Methods for Improving Spherical Correction. By Prof. Ahhe. 823 



would not need a separate correcting leng, as this could be com- 

 bined with the concave " amplifier " introduced by Dr. Woodward 

 for preserving the right position of the conjugate foci, while the 

 image is projected to a considerable distance. This amplifier may 

 be readily made use of for correcting every difference of amplifica- 

 tion, provided it be not made achromatic, but rather over-corrected 

 chromatically to a proper degree. 



Some difficulties, arising from the nature of the immersion 

 fluid, which at first occasionally interfered with the satisfactory 

 performance of the objectives in a rather vexatious manner may now 

 be considered as overcome. As the difierence in the refractive 

 index between oil of cedar- wood and the covering glass seemed to 

 me too great for the increased aperture and as the stronger oils or 

 mixtures of oils are of too great dispersive power, I was obhged to 

 abandon these liquids. The solution of anhydrous chloride of zinc, 

 which can be easily concentrated up to the refraction of covering 

 glass, by using a refractometer or a test-bottle with a crown-glass 

 prism of 1-518, at first appeared to be the best substitute. But 

 it was found that in the high concentration needed here the solution 

 is subject to sudden changes unless it is perfectly free from the 

 ordinary hydrous salt. This may be obtained by very careful 

 preparation, and the liquid then performs very well — pro^dded it be 

 preserved in a tight bottle and prolonged observation with the same 

 drop be avoided, but I prefer now a solution of chloride of cad- 

 mium, or of sulpho-carbolate of zinc, in strong glycerine, which can 

 be brought to the right refraction. They are rather sticky it is 

 true, and in this respect less convenient for use ; but as they are 

 not nearly so hygroscopic as the chloride of zinc, they can be 

 applied with much more ease. With sufficient cleanliness none of 

 these liquids, not even the chloride of zinc, will do any injury to 

 the lenses or to the stands. The application of strictly homo- 

 geneous immersion has, therefore, no serious drawback, even if 

 a more convenient liquid should not be hereafter discovered, as may 

 still be hoped.* 



As the result of the practical trial here recorded, I consider 

 the three following facts as sufiiciently established : — 



1. The chromatic difference of spherical aberration may be 

 overcome even in wide-angled systems by means of the ordinary 

 methods hitherto applied in optical construction. 



2. By the removal of this defect of spherical correction in 



* Microscopists who are connected with chemical matters are kindly requested 

 to promote the homogeneous immersion method by looking out for such less 

 known liquids as afford some hope of being useful for the purpose, and I shall be 

 glad to investigate every sample which may be sent to me tor trial. A few drops 

 only, is quite sufficient for exact measurement, by means of the refractometer, of 

 the refractive and dispersive indices. 



