ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 855 



ticular immersion fluid, in consequence of variations in temperature ; 

 or of an alteration in the optical properties of the cover-glasses, 

 and the different powers of accommodation of the eyes of different 

 observers. 



The author has already shown * the practical insignificance of the 

 difference in the refractive index of the immersion fluid produced by 

 the varying temperature of the observing room in the ordinary use of 

 the objectives in question, where the changes of temperature cannot be 

 very important. Theoretically considered also, the matter will be 

 seen to be of ouly little moment. According to the measurements of 

 Professor Abbe, the difference with cedar oil is but 0*008 for a 

 variation in temperature of 3° C. Since the correction of the ob- 

 jectives is arranged for a medium temperature of from 18° to 20° C, 

 and the temperature at which normal microscopical observations are 

 made is certainly (even if we allow very wide limits) between 15° and 

 28° C, the greatest deviation from the mean value in the refractive 

 index is at most two or three units in the third decimal place. The 

 aberrations in the divergence of the incident rays connected with 

 this slight change and the consequent disturbance of the spherical cor- 

 rection, whilst it can be demonstrated by very accurate testing on the 

 silver plate, is nevertheless in any case much smaller than those devia- 

 tions from the best correction which occur with the correction-collar. 

 It therefore follows that this much enforced deviation in the refractive 

 index of the immersion fluid, caused by variations of temperature, 

 which is to be balanced by the correction-collar, is at all events the 

 lesser of two evils, and consequently can furnish no pretence for 

 doing away with the fixed mounting. 



Still less than the above-mentioned variations can the differences 

 in the refractive index of different cover- glasses give any inducement 

 for the introduction of the correction-collar. According to the 

 observations of Professor Abbe during a period of ten years, these 

 differences are so extremely small that they may be regarded 

 practically as nil. 



Finally, the suggested influence of the different powers of ac- 

 commodation of the eye must be relegated to the region of dreams, as 

 a simple theoretical consideration will show. If we take for example 

 a power of 800, and two observers whose eyes are accommodated 

 respectively to 100 mm. and an infinite distance, the difference in 

 the adjustment thus produced — that is in the actual object-distance, 

 assuming the objective to have air on both sides of it — can be easily 

 computed from the formula : — 



x x* = - f 2 . 

 For a long-sighted eye (where x* = oo) 



x = 0. 



For a distance of vision of 100 mm. (where x* = — 100) 



f 2 250 



x = i- — , and since in the Microscope as a whole / = ■==- ■> 



100 L J N 



" ' X \ N ) 100 



* Bot. Centralbl., No. 6. 



