Miscellaneous, 471 



1. The first step, we conceive, is, to abandon large angular aper- 

 tures, and to use object-glasses of moderate focal length, obtain- 

 ing at the eye-glass any additional magnifying power that may 

 be required. 2. In order to obtain a better illumination, either 

 by light incident vertically or obliquely, a new form of the mi- 

 croscope would be advantageous. In place of directing the mi- 

 croscope 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 opaque 

 or transparent, and may subject it to any experiments he may 

 desire to make upon it. It may thus be studied without a cov- 

 ering 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 modification. If glass is to be 

 used for the lenses or 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, how- 

 ever, that the time is not distant when transparent 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 

 refractions may be removed by cutting it in plates perpendicular 

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

 erally speaking, less perfect along ths axis of double refraction 

 than in any other direction, but this imperfection does not exist 

 in topaz. — Prof. Stokes and Mr. Stoney suggested some modifi- 

 cations of Sir David Brewster's theoric views ; and a member of 

 the Section whose name we did not catch, stated that several 

 attempts had been made to form an image of objects more re- 

 moved from the first or object glass of the microscope than at 

 present, by using an additional lens, but hitherto without success. 



