which means he daunted the enemy. He had a statue raised to his honour in that same posture.

'Myrtillus's shield, it is said, secured him in the field, and saved him when shipwrecked at sea, by wafting him to the shore. But how much more serviceable is the shield of faith! By this the Christian overcomes his spiritual enemies, and is enabled to triumph even in the midst of difficulties. Such at last will have to say, "I have fought a good fight, I have kept the faith. Henceforth there is laid up for me a crown of righteousness, and not to me only, but to all them who love his appearing." p. 333.

Art. X. Preparatory Prayers, and a Companion to the Altar. By a Member of the Church of England. The Second Edition, with Additions. 24mo. price 1s. 6d. bound. Darton and Co. 1814. THE grand requisite of forms of prayer is, that they should

express the feelings of those who are to use them. To put into the mouths of others the expressions of a penitence to which they were never subdued, or the fervours of a devotion into which they were never kindled, is not merely absurd; it is profane. It is to make them either formalists or hypocrites. That prayers then may thus express the feelings of him who uses them, it is necessary, that they express the feelings of him who writes them,-that they be the work not of the fancy, but of the heart. It is an old remark concerning poetry, that what comes from the heart goes to the heart and the same is true in devotion. There is such a similarity in the feelings and affections, and wants, and wishes of Christians, in their acknowledged weaknesses and sins, in the mercies they receive from God, that, allowing for constitutional differences, and peculiar circumstances, they can make use of the same confessions, the same prayers, and the same thanksgivings.

As the feelings expressed must be genuine, the language in which they are expressed should be simple. It was a command in the Mosaic ritual, that the altar of God should be without carved work-of earth, or of unhewn stone; if thou lift up thy tool upon it, thou hast polluted it.' Ex. xx. 25. In fact, if the first rule be observed, the second cannot be infringed. True feeling always will unburthen itself in plain words. He who is bewailing his sins, and imploring pardon, has no thoughts to spend upon the tricks of rhetoric,

The little work before us consists of prayers for every morning and evening of a week of preparation for the Lord's Supper, together with directions' for rightly and duly' partaking of that holy sacrament, and the service appointed for it by the church. The composition is so uniform throughout, that we may save ourselves any observation thereupon, by extracting one of the prayers at length,

A Prayer to be used in Church, as soon as the Morning Service is ended...

O most Blessed Lord, who of Thy great mercy hast given Thine only begotten Son to be a sacrifice for the sins of Thine unworthy servants grant that this Thine inexpressible love, may not be lost unto me; but that being sensible of my sad condition by nature, and my worse condition by individual sin, I may be thoroughly convinced of the necessity, and great blessing, of a Redeemer: that so I may, with a heart filled with thanksgiving and godly love, most fervently join with this Thy congregation, in renewing the remembrance of what Thy dear Son has done and suffered for us; of His bitter cross and passion; His glorious resurrection and ascension; and of His coming again in majesty, to judge the quick and the dead. Give me, O God, a stedfast faith in Thy promises through Him, and a firm trust in Thy Almighty power. Let the fear of Thy justice and omniscience keep me from presumptuous sins, and a sense of Thy goodness and mercy preserve me from despair. Guard me from all coldness, indifference, and carelessness in religious duties; particularly in that which I am now about to perform. I know I am not worthy so much as to gather up the crumbs which fall from Thy table; but I come not, presuming on my own righteousness; it is in obedience to Thy commands, O most blessed Saviour, and trusting in Thy love. Blot out mine iniquities, cleanse me with Thy holy spirit, fulfil me with Thy heavenly grace, and receive this my sacrifice, O God, for Thy dear Son's sake, Who liveth and reigneth with Thee and the same Spirit, one God, for evermore. Amen.'

Art. XI. A Treatise on New Philosophical Instruments, for various purposes in the Arts and Sciences. With Experiments on Light and Colours. By David Brewster, LL. D. Fellow of the Royal Society of Edinburgh, and of the Society of Antiquaries of Scotland. 8vo. pp. xx. 427. 12 folding plates. Price 18s. London, Murray; Edinburgh, Blackwood. 1813.

WE regret much that we have suffered this ingenious

work to lie by us so long unnoticed, and that even now we can notice it but briefly. It is divided into five books, of which the first relates to micrometers; the second, to instruments for measuring angles when the eye is not at their vertex; the third, to telescopes and other optical instruments for measuring distances; the fourth, to optical instruments for different purposes, in which the rays are transmitted through fluids; and the fifth, to new telescopes and microscopes. The new instruments which are described in this last book, amount to more than thirty; and though, from the nature of things, a greater degree of ingenuity and skill is exhibited in the invention of some of them than in that of others, there is scarcely one which is

not the result of such a combination of talent and judgement as does not fall to the lot of ordinary men. In the invention of these instruments, a very extensive acquaintance with optical theories is blended with an accurate knowledge of mechanical constructions. Few of them, however, can be thoroughly comprehended without diagrams; and therefore we refer the curious in these points to Dr. Brewster's treatise.

Our author does not, however, present himself to the public merely as an inventor of ingenious and useful instruments, but also as the discoverer of new and interesting properties. In the fourth book, which we regard as the most valuable part of this treatise, we have the deseription of an instrument for measuring the refractive powers of fluids, and a method of determining the refractive powers of solids, to which are added seven tables: containing 1, the refractive powers of 215 solid and fluid substances. 2. Refractive powers of phosphorus, sulphur, and 35 other substances. 3. Refractive powers of the fluids of a young haddock's eye. 4. Refractive powers of the fluids of a lamb's eye. 5. Refractive powers of muriate of antimony under different circumstances. 6. Refractive powers of 16 vegetable juices in different circumstances. 7. Refractive powers of precious stones and other minerals. From the experiments of which the results are here tabulated, Dr. Brewster was naturally led to others upon Dispersive powers, a subject which presented a series of new and interesting results. With the aid of a new instrument, described in the 3d chapter of book iv., he measured the dispersions of 137 substances, of which nearly 100 had never been examined before, and determined by calculation the absolute dispersive powers of those several substances. The uncorrected colour which remains after equal and opposite dispersions, induced the doctor to examine carefully the action which different bodies exercise upon the differently coloured rays. The numerous experiments which he made with that view, while they establish this difference of action, and prove the existence of a tertiary spectrum, suggest some principles which may contribute to the improvement of the chromatic telescope. This train of inquiry led to some of our scientific experimenter's discoveries, the nature of which he has clearly indicated in small compass, as follows:

The discovery of a new property impressed upon light, by transmission through the agate, opened a still wider and more alluring field of enquiry; and though this subject was not immediately connected with the description of any instrument, I prosecuted it with renewed zeal, and examined the variation which light, thus modified, experienced from the action of reflecting and reflecting substances. The power of transparent bodies to destroy this property; the optical phenomena peculiar to mica and topaz; and the singular

alternations of the prismatic colours which these bodies impress upon polarized light, were thus established by numerous experiments.

The leading results which were obtained in the course of these researches may be thus enumerated.

1. It has been ascertained that chromate of lead and realgar have a greater refractive power than the diamond, which has always been supposed to exceed every other body in its action upon light.

2. The chromate of lead possesses a double refraction, about thrice as great as that of Iceland spar.

3. The three simple inflammable substances have their refractive powers in the very order of their inflammability.

4. All doubly refracting crystals possess a double dispersive power, the greatest refraction being accompanied with the highest power of dispersion.

'The fluates, viz. fluor spar and cryolite, have the lowest refractive powers of all solid substances, and the lowest dispersive powers of all bodies.

6. The agate, when cut by a plane at right angles to the lamina of which it is composed, impresses upon a transmitted ray of light the same character with one of the pencils formed by doubly refracting crystals.

7. This property of light, whether communicated by the agate, or by double refraction, or by reflection from transparent bodies, may be destroyed by transmitting the light, in one direction, through almost all mineral substances, and even through horn, tortoise shell, and gum arabic; while in another direction the original character of the ray is not altered. The axis of the substance in which the property is destroyed, I have called the depolarizing axis; and the axis in which it is not altered, the neutral axis.

8. Mica and topaz, while they possess in common with other bodies, the neutral and depolarizing axis, have also axes of different kind. Each depolarizing axis of the mica is accompanied with an oblique neutral axis, while the neutral axis, between the two common depolarizing axes, has an oblique depolarizing axis.

9. When the images of a luminous object are depolarized by the mica, they exhibit, by a gentle inclination of the plate, the most singular alternations of the prismatic colours. The same colours were observed in the topaz; and, in a more perfect manner, in a rhamboid of Iceland spar, which exhibited some new phenomena.

10. Light suffers a peculiar modification when reflected from the oxidated surface of polished steel, which seems to prove that the oxide is a thin transparent film.

11. Light is partially polarized when reflected from polished metallic surfaces.

12. The light reflected from the clouds, the blue light of the sky, and the light which forms the rainbow, are all polarized.

13. It appears, from a great variety of experiments, that bodies exert a different action upon the different coloured rays, oil of cassia having the least, and sulphuric acid the greatest, action upon green light.

14. The existence of a third, or a tertiary spectrum, has been established by numerous experiments; and a method has been pointed out of employing this spectrum as a measure of the action which different bodies exercise upon the differently coloured rays.'

For a full account of the train of investigation which led to these curious results, we refer the scientific reader to the volume before us. Dr. Brewster has pursued still farther this interesting branch of inquiry. A portion of his results is exhibited in the London Philosophical Transactions for 1813, and will come under our review when we speak of that volume. A subsequent portion is described very concisely by Dr. Brewster himself, in the third volume of Dr. Thomson's "Annals of Philosophy," from which we give the following quotation:

I have found that light transmitted obliquely through all bodies, whether crystallized or uncrystallized, suffers polarization like one of the pencils produced by double refraction; and from a great number of of experiments, I have been enabled to determine the law by which all the phenomena are regulated.

If light is incident at any angle, except a right angle, upon the surface of a transparent body, a portion of the transmitted pencil will suffer a polarization. The quantity of polarized light varies as the cotangent of the angle of incidence; and there is always a particular angle, depending on the refractive power of the body, at which the emergent light is wholly polarized. When the light is transmitted necessarily through several parallel plates, either in contact or at a distance, the cotangents of the angles of polarization are always to one another as the number of plates employed; and the number of plates multiplied by the tangent of the angle at which they polarize light is a constant quantity. If the angle of incidence exceeds the angle of polarization, the pencil will still emerge in a polarized state.

A parcel of 8 plates of plate glass polarizes the transmitted light at an angle of 79° 11', and at any angle of incidence greater than this.

A parcel of 16 plates polarizes the light at any angle above 69° 4', and

A parcel of 47 plates at any angle above 41° 41'.

Similar effects, varying however with the refractive power, are produced by plates of mica, by films of blown glass, by coats of grease, gold-beaters' skin, and even gold leaf itself."

Malus's discovery of the polarization of light by reflection is, perhaps, one of the most brilliant discoveries that optics has ever received; but though it developed a new set of phenomena analogous to those produced by doubly refracting crystals, yet as the polarization was obviously effected by reflection, and not by refraction, it did not furnish any information respecting the methods by which these crystals polarized the transmitted light. The discovery, however, of the polarization of light by oblique refraction forms the connecting link between these two classes of phenomena, and holds out the prospect