ACHROMATIC OBJECTIVES 19 



one kind of glass is proportional to their dispersion in the other, 

 then when any two are destroyed the third is destroyed with them. 

 This unfortunately is not the case in practice, because two kinds of 

 glass having proportional dispersion powers cannot be obtained. 

 This, however, is what really happens. G may lie midway between 

 R and Y in one kind of glass, but in the other it may lie, for 

 instance, much nearer R, say a third instead of half the distance 

 of R from Y. If now the dispersion of R Y be destroyed, G will 

 be left outstanding. If a different angle of prism be chosen, so that 

 R and G are neutralised, then Y must be left outstanding. 



This want of proportion in the dispersion of the various colours 

 of the spectrum in two kinds of glass is termed the irrationality of 

 the spectrum, and the colour or colours left outstanding in a corrected 

 combination of lenses is known as the secondary spectrum. 



In some subsequent pages we shall have to call attention to the 

 manufacture in Germany of some new vitreous compounds by the 

 combination of which with fluor spar the secondary spectrum has 

 been removed from microscope objectives, and an apochromatic 

 system of construction has been introduced. 



Meanwhile, we may remember that it has only been in compa- 

 ratively recent times that the construction of achromatic object- 

 classes for microscopes has been brought about, but the gradual 

 enlargement of aperture and the greater completeness of the cor- 

 rections soon after the discovery of achromatism rendered sensible 

 an imperfection in the performance of these lenses under certain 

 circumstances, which had previously passed unnoticed, and Andrew 

 Ross made the important discovery that the use of cover-glass in 

 mounting minute objects introduced aberration, and that a very 

 obvious difference exists in the precision of the image, according as 

 it is viewed with or without a covering of thin glass, an object- 

 glass which may be perfectly adapted to either of these conditions 

 being sensibly defective under the other. 



He also devised the means of correcting this error, and published 

 his device in vol. li. of ' Transactions of the Society of Arts ' for 1837. 



Fig. 21 will illustrate the effect produced on the corrections of 

 an object-glass by the interposition of a cover-glass between the 

 object and the objective. 



The rays radiating from the object O in every direction fall upon 

 the cover-glass C C (/j = 1*6). On tracing two definite rays, such 

 as O A and O B, it will be found that they will be refracted to R 

 and P (shown by the dotted lines of the figure). On their emergence 

 into air they will be again refracted in a direction parallel to their 

 first path, and will enter the front lens of the objective at the 

 points M and N. 



Now as M R and N P, produced, meet in Y, it follows that, so 

 far as the objective is concerned, the rays M R, N P might have 

 diverged from the point Y. 



Similarly, by tracing two of the less divergent rays from O they 

 will be made by the refraction of the cover-glass to appear as if 

 they diverged from X. Therefore, in consequence of the cover-glass 

 the objective has to deal with rays radiating apparently from two dis- 

 ci 



