260 Transactions of the Sociefy. 



cedar- wood oil (prepared by Scliimmel and Co., Leipsic and New 

 York) an essential oil almost without colour or smell, and not 

 volatile, but unfortunately rather thin. Its refractive index at a 

 medium temperature is about 1"51, whilst the dispersion only 

 slightly exceeds that of crown glass. The objectives have therefore 

 been constructed for use with this oil. 



For a more extended application of the principle of homogeneous 

 immersion great advantage is derived from the fact, that by mixing 

 one of the more highly refracting essential oils, such as oil of cloves, 

 fennel, aniseed, or others, with a certain quantity of olive oil, 

 fluids can be readily obtained which are equal to cedar- wood oil 

 in refractive power, but whose dispersive power may be increased 

 more or less, as required. This provides a means of regulating the 

 chromatic correction of greater delicacy than is attainable by any 

 mere mechanical correction, inasmuch as for cedar- wood oil can be 

 substituted mixtures of various dispersive power, according to the 

 nature of the object to be examined and the kind of illumination 

 required. By this simple means, for example, the chromatic 

 difference of spherical aberration, a correction -de feet which (in the 

 present state of practical optics) it is impossible to overcome in 

 objectives of large aperture, is rendered for the most part immaterial. 

 This unavoidable defect is apparent from the fact that the central 

 and peripheral zones of the objective are never simultaneously 

 perfectly achromatic. An objective which with oblique light gives 

 an image as free from colour as possible, is found, when central 

 illumination is used, to be chromatically under- corrected to a marked 

 degree, in the case of a sensitive object, and conversely. This is the 

 more striking the larger the angular aperture. If, now, instead of 

 a stratum (with parallel surfaces) placed in the course of the rays 

 we substitute another of equal refractive but different dispersive 

 power, we obtain a simple means of changing the chromatic cor- 

 rection of the objective without altering the spherical correction, 

 and if, as is done throughout in the construction of these lenses, 

 the chromatic compensation is so arranged that the fluid having 

 the lowest dispersion (cedar-wood oil) produces the best achroma- 

 tism for oblique light, the use of a more highly dispersive mixture 

 of the kind mentioned will correct the chromatic defect for central 

 illumination which would otherwise appear. 



The application of this method is adversely affected by one 

 circumstance only, viz., that the effect of a determinate increase in 

 the dispersion naturally depends upon the thickness of the fluid 

 stratum. With covering glasses of different thickness, as also 

 with objectives of different focal lengths and corresponding different 

 working distances, one and the same mixture will yield more or less 

 unequal results. 



Since the exact adjustment of the immersion fluid thus appears 



