182 Researches in Circular Solar Spectra. 



On the Aberrations of Ei/e-jpieees, with Suggestions for forming a 

 Comjjensating Eye-piece for Microscoj^es and Telescopes ; on the 

 Priyiciple of searching the Axis for Aplanatic Images. 



The Astronomer Koyal has given an account of a trial of 

 several kinds of eye-pieces, and some of their bad effects.* 



The use of a solar disk formed by an eye-piece fixed close to the 

 prism-heliostat deprived of its lens, and examined by the method 

 already described by means of accurately corrected objectives, places 

 the achromatism of the eye-piece under a severe scrutiny. Very 

 rich and beautiful colours are developed in the solar rings (pre- 

 viously obtained as pale as possible), corresponding to the extent of 

 the chromatic errors, A Huyghenian eye-piece was placed close to 

 the heliostat so as to form a brilliant disk of the sun ; the adjusted 

 spectrum apparatus immediately flashed with brilliant colom^ed 

 rings, before this, appearing pale lavender and white. 



During the use of the searcher for aplanatic images, it occurred 

 to me to investigate the efi'ects of pushing the eye-piece gradually 

 nearer the object-glass without a searcher. 



I discovered that, when within 4 inches, the definition showed 

 violent under-correction. 



I now conceived the idea of substituting a traversing movement 

 of the eye-piece, especially for glasses unprovided with a Koss collar, 

 as a correction for thickness of cover. 



A very firm sliding tube was constructed, and I now found I 

 had substituted a range of several inches, as a correction, for that 

 of a few hundredths of an inch used in the Koss adjustment. 



Experiment. — Adjusting the apparatus and the screw collars of 

 the objectives for severe testing, a bunch of small glass drops, of 

 diameter • 04 inch, was suspended in front of the heliostat so as to 

 present a minute image of the sun. The searching eye-piece being 

 placed at 10 inches distance from the stage of the microscope, in 

 the plane of which the solar disk is formed, and the minute solar 

 disk observed in a state of balanced corrections, it was found that 

 as the eye-piece was traversed towards the disk it became gradually 

 more and more under-corrected. 



It now became evident that this movement was, upon a large 

 scale, equivalent to the eff'ect of the Eoss collar movement upon a 

 minute scale. 



It now occurred to me, as a thick cover and a water film over- 

 corrected an objective, that a dry objective might in many cases be 

 transformed into an " immersion " simply by advancing the eye- 

 piece ; also that a sufficient variation of interval between the front 

 lenses might in many cases enable a dry lens to act as an immersion. 



[The immersion principle is valuable for the increased volume 

 * Astronomical notices. 



