84 PHOTO-MICROGEAPHY 



object-glass to gather up the whole of the rays that were lost when the ray entered 

 air, as shown on the other side of the diagram instead of the oil. It is obvious, then, 

 why immersion lenses give so much more light. 



The same class of reasoning holds good with the mounting media of specimens. If 

 they are mounted dry, which means in air, many diffractive beams which pass from 

 the object, all of which, theoretically, if it were possible, should be caught up by the 

 objective, are lost ; whereas, if mounted in Canada balsam (refractive index, 1*5), many 

 more may be caught. 



It is very obvious, then, that, theoretically, dry objectives cannot pass so much 

 light as immersion ones, and this is practically found to be correct. What is more to 

 the point, the argument also shows that the limit of dry objectives is N. A. i 'o, because 

 all the outside rays will be lost without the interposition of some homogeneous 

 substance to optically join up the gap between the objective and the cover-glass. It 

 is for this reason that immersion lenses are often called homogeneous systems. 



Another point not to be forgotten is that as the cover-glasses vary in thickness 

 and density, so the bending of B D may vary in direct accordance, and it is to obviate 

 this that dry lenses of high N. A. are provided with a " cover-glass adjustment," 

 which so regulates the performance of the objective as to accommodate it to the 

 differences in question. To practise the user to arrange the adjustment, as well as 

 for other reasons, the firm of Zeiss sell a slip upon which several cover -glasses of 

 known thickness are placed, and as the firm mark the adjustments for different 

 thickness of cover-glass on the objective itself, it enables the beginner to practise 

 over and over again the art of getting his adjustment correctly by sight, and then 

 proving his result, until he is quite an adept, by comparing the figures on the cover- 

 glass with those on his objective. This object lesson, which re([uires some patient 

 practice and learning, is of great service, especially to the photo-micrographer, as it 

 will be seen hereafter when speaking of depth of vision in lenses (page 87). It will 

 then be shown a great deal of accurate vision is dependent on the accommodation 

 of the eye — hence the greater amount of accuracy in focussing, &c., is required 

 when photographing an object where the accommodation of the eye does not come 

 into play, and every adjustment to improve such focussing must be employed. 



Inasmuch as the presence of the cedar oil forms a homogeneous system, so it is 

 obvious that any small variation in thickness of cover-glasses does not make any 

 difference ; hence the majority of immersion lenses have no cover-glass adjustment. 

 Messrs. Powell & Lealand, however, still consider that, notwithstanding what has 

 been said, the very finest of definition under certain circumstances may be only 



