INFLUENCE OF THE COVER-GLASS. 67 



It is evident from what has been stated (pp. 60, 61), as to the 

 path of the marginal rays, that a diminution of the object-distance, 

 which in the case of rays of 40 C inclination amounts to about 

 ^ mm. more, even with a cover-glass of only i mm., must exercise 

 a noticeable influence on the position of the corresponding objec- 

 tive-image, and, at the same time, on the correction of the aberra- 

 tions. An objective which produces sharp images of uncovered 

 objects will, on the application of a cover-glass, define the more 

 indistinctly the greater the alteration of the object-distance 

 thereby caused. Similarly, an objective intended for immersion 

 in water will prove less aplanatic for observations in air, unless 

 the error is rectified by alteration of the distances of the lenses. 

 From this it is evident that objective-systems with angular 

 apertures of 120 to 160, as now supplied by opticians, are far 

 more sensitive to such influences than those in which the angles 

 of aperture amount to only 60 or 80. On this account the high- 

 power objectives of most recent construction are furnished with 

 means to alter the distance of the front lens from the two others, to 

 the extent necessary for proper compensation. As a rule, the 

 lenses must be the nearer together, the greater the apparent 

 approximation of the object caused by the cover-glass. Contriv- 

 ances for effecting this compensation are applied to the high-power 

 objectives of various German, French, and English makers. The 

 mechanism is shown in Fig. 27. The metal stud s indicates the 

 different positions of the lenses. The % 

 medium position a, and the two extreme ^ r-, 



positions b and c, are shown separately. ^ 



The approximation of the lenses should be 

 effected preferably in such a way that the 

 front lens remains fixed, instead of move- 

 able, as formerly, which is attended by the 

 practical advantage that the object does 

 not disappear from the field of view during the adjustment. 



Mohl has written in detail in his " Mikrographie " upon the 

 influence of cover-glasses on objectives of medium power, and we 

 refer to his statements with regard to the older systems of Ploeasl 

 and Amici. With regard to the recent instruments which we 

 have examined, we have not found that the differences of thick- 

 ness utually met with in cover-glasses (say J to J of a millimetre) 

 influence the microscopic image to any considerable degree. 



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