NEW VITBEOUS OPTICAL COMPOUNDS 31 



High refractive media can greatly reduce the value of even the 

 wave-length of light, and what is possible in the production of vitreous 

 combinations, refractive fluid media, and mounting substances we 

 may not forecast ; but, judging from the past, we have by no means 

 reached their limit. At the same time, it may be remembered that 

 photo-micrography, by constantly covering a wider area of applica- 

 tion with its ever increasingly delicate and subtle methods, is 

 more penetrating in the revelation of structure than the human 

 eye. 



It may be taken for granted that- in the present state of optical 

 mathematics the opticians, English, Continental, and American, 

 have given up the quest of many things fruitlessly sought. Empty 

 amplification is a folly of lenses of the past. Magnification without 

 concurrent disclosure of detail is of no more scientific value for the 

 .disclosure of structure than the projection of the photo-micrograph 

 by an electric arc upon a screen would be. What is needed is an 

 ever-increasing exactitude in the formation of the dioptrical image. 

 The imperfection of this at the focal point springs from two causes : 

 one, as we have just demonstrated, arises from the residual spherical 

 and chromatic aberrations, the other takes origin in the want of homo- 

 geneity, absolute precision of curve, and perfect centering of the system 

 of lenses in a combination. This causes the cone of rays proceeding 

 from the object to unite, not in perfect image points, but in * light 

 surfaces of greater or less extent circles of dissipation ' which 

 limits the distinctness of minute details. It is the faults of the ob- 

 jective that in practice are alone important, and with the crown and 

 flint glass commonly at the disposal of the optician there are two 

 great drawbacks to perfection, or rather to an approximation to it. 



1. The first arises from the unequal course of the dispersion in 

 crown and flint glass, already described, which makes it impossible 

 to unite perfectly, with the properties they possess, all the coloicred 

 rays in an image. Absolute achromatism cannot by their means be 

 attained, the dispersion at different parts of the spectrum being so 

 greatly disproportional. It has never been possible to unite more 

 than two different colours of the spectrum. The rest, in spite of all 

 effort, deviate and form the secondary spectrum, leaving, in the very 

 finest lenses, circles of dispersion not to be excluded. 



2. The second defect arises in the impossibility of correcting by 

 means of ordinary crown and flint glass the spherical aberration for 

 more than one colour. If the spherical aberration be removed as 

 far as may be for the centre of the spectrum, there remains under- 

 correction for the red, and over-correction for the blue and violet 

 rays, presenting a want of balance between the chromatic corrections 

 for the central and marginal zones of the objective. Although 

 perfect chromatic corrections for the central rays may be effected, 

 giving images of great beauty, the chromatic over-correction for the 

 peripheral rays with oblique illumination will show the borders of 

 the image with distinct chromatic fringes. 



To compensate these aberrations in the construction of an object- 

 glass, what is needed is a vitreous material applicable to optical 

 purposes possessed of such properties that a relatively smaller re- 



