TRANSACTIONS OP THE SECTIONS. 9 



so close to the object-glass. 2. There is a great loss of light, from its oblique in- 

 cidence on the surface "of the first lens. 3. The surface of glass, — with the most 

 perfect polish, — must be covered with minute pores, produced by the attrition of 

 the polishing powder ; and light, falling upon the sides of these pores with extreme 

 obliquity, must not only suffer diffraction, but be refracted less perfectly than when 

 incident at a less angle. 4. When the object is almost in contact with the anterior 

 lens, the microscope is wholly unfit for researches in which mechanical or che- 

 mical operations are required, and also for the examination of objects enclosed in 

 minerals or other transparent bodies. 5. In object-glasses now in use, the rays 

 of light must pass through a great thickness of glass of doubtful homogeneity. It 

 is a question yet to be solved whether or not a substance can be truly transparent, 

 — in which the elements are not united in definite proportion, — in which the sub- 

 stances combined have very different refractive and dispersive powers ; and in 

 which the particles are so loosely united that they separate from one another, as in 

 the various kinds of decomposition to which glass is liable. 



If the best microscopes are affected by these sources of error, every exertion 

 should be made to diminish or remove them. 1. The first step, we conceive, is 

 to abandon large angular apertures, and to use object-glasses of moderate focal 

 length, effecting at the eye-glass any additional magnifying power that may be re- 

 quired. 2. In order to obtain a better illumination, either by light incident verti- 

 cally or obliquely, a new form of the microscope would be advantageous. In place of 

 directing the microscope to the object itself, placed as it now is almost touching 

 the object-glass, let it be directed to an image of the object, formed by the thinnest 

 achromatic lens, of such a focal length that the object may be an inch or more 

 from the lens, and its image equal to, or greater, or less than the object. In this 

 way the observer will be able to illuminate the object, whether opake or trans- 

 parent, and may subject it to any experiments he may desire to make upon it. It 

 may thus be studied without a covering of glass, and when its parts are developed 

 by immersion in a fluid. 3. The sources of error arising from the want of perfect 

 polish and perfect homogeneity of the glass of which the lenses are composed, are, 

 to some extent, hypothetical ; but there are reasons for believing,— and these 

 reasons corroborated by facts, — that a body whose ingredients are united by fusion, 

 and kept in a state of constraint from which they are striving to get free, cannot 

 possess that homogeneity of structure, or that perfection of polish, which will allow 

 the rays of light to be refracted and transmitted without injurious modifications. If 

 glass is to be used for the lenses of microscopes, long and careful annealing should 

 be adopted, and the polishing process should be continued long after it appears 

 perfect to the optician. We believe, however, that the time is not distant when trans- 

 parent minerals, in which their elements are united in definite proportions, will be 

 substituted for glass. Diamond, topaz, and rock-crystal are those which appear 

 best suited for lenses. The white topaz of New Holland is particularly fitted for 

 optical purposes, as its double refraction may he removed by cutting it in plates 

 perpendicular to one of its optical axes. In rock-crystal the structure is, generally 

 speaking, less perfect along the .axis of double refraction than in any other direc- 

 tion, but this imperfection does not exist in topaz. 



On the decomposed Glass found at Nmeveh and other places. 

 By Sir David Brewster, K.H., JF.R.S. 



The different kinds of glass which are in common use, consist of sand or silex 

 combined by fusion with earths or alkalies, or metals which either act as fluxes, or 

 communicate different colours or different degrees of lustre or refractive power to 

 the combination. 



In quartz or rock-crystal, which is pure silex, and in other regularly crystallized 

 bodies, the molecules or atoms unite in virtue of regular laws, the pole of one atom 

 uniting with the pole of another. Such substances, therefore, do not decompose 

 under the ordinary action of the elements. The lens of Hock- Crystal, for example, 

 found bj r Mr. Layard at Nineveh, is as sound as it was many thousand years ago 

 when in the form of a crystal. 

 ■ In the case of glass, however, the silex has been melted and forced into union 



