74 The Microscope. 



the same objectives when used by central and then by oblique 

 light, Professor Abbe immediately recognized the advisability 

 of using two immersion fluids with the homogeneous immersion 

 objectives, constructed by Zeiss in fixed mountings. The fluid 

 for oblique light was as near as possible, optically homogeneous 

 with the glass of which the front lens was made; while that 

 for central light work, while having a mean refraction to that 

 of the front lens, exceeded it in dispersive power — any slight 

 disturbance of the spherical correction being easily compen- 

 sated for by a change in the length of the body by the use of 

 the draw-tube. If these conditions be properly attended to, 

 there is but little choice (optically) between the two forms of 

 objectives ; but when, as is frequently the case, it becomes nec- 

 essary to use a fluid having a considerably lower power of mean 

 refraction than the homogeneous fluid, such as water or glycer- 

 ine, the collar is a great advantage, as the range of action of 

 the draw-tube is not sufficient to produce the negative correc- 

 tion requisite with fluids of such low density. 



Many optiicans have made objectives upon the homogene- 

 ous immersion principle, of very large aperture and in fixed 

 mountings, which have only been provided with fluid for ob- 

 lique light. Hence it is, that on account of their large aper- 

 tures and proper corrections, their oblique, light work has been 

 remarkably fine ; but when used with the same fluid by central 

 light, the chromatic under correction has been so great as to 

 injure the definition to a considerable extent. 



If the corrections could be easily made, so that there would 

 be no perceptible difference between central and oblique light 

 work, and consequently only one fluid be required, these ob- 

 jectives would be much more convenient for those who, while 

 requiring the best results possible, do not care to be bothered 

 by the constant changing of fluids. That such a condition is 

 possible, in the construction of these objectives, is shown by a 

 new -J recently produced by the Spencers. The difference in 

 both chromatic and spherical aberrations in this objective for 

 the central and peripheral zones of the lenses, have been re- 

 duced to such a small residuum that there is practically no 

 difference — at most not enough to interfere with good definition, 

 as is shown by the lens upon Moller's test plate ; in which the 



