1873.] 



Circular Solar Spectra. 



435 



colour be made to disappear. A white centre and exceedingly black 

 rings, interspaced with a pale lavender and rose-colour, were the nearest 

 approaches to perfect achromatism which I could produce. 



On the Aberrations of Eyepieces, iviih suggestions for forming a Compen- 

 sating Eyepiece for Miscroscopes and Telescopes, on the principle of 

 searching the axis for Aplanatic Images. 



The Astronomer Eoyal has given an account of a trial of several 

 kinds of eyepieces and some of their bad effects *. 



The use of a solar disk formed by an eyepiece 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 eyepiece under a severe scrutiny. Very rich and 

 beautiful colours are developed in the solar rings (previously obtained 

 as pale as possible), corresponding to the extent of the chromatic errors. 

 A Huyghenian eyepiece was placed close to the heliostat, so as to form a 

 brilliant disk of the sun ; the adjusted spectrum-apparatus immediately 

 flashed with brilliant-coloured rings, before this appearing pale lavender 

 and white. 



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

 to investigate the effects of pushing the eyepiece gradually nearer the 

 object-glass without a searcher. 



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

 violent undercorrection. 



I now conceived the idea of substituting a traversing movement of the 

 eyepiece, especially for glasses unprovided with a Eoss 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 Eoss adjustment. 



Experiment. — Adjusting the apparatus and the screw collars of the ob- 

 jectives for severe testing, a bunch of small glass drops, of diameter 0*04 

 inch, was suspended in front of the heliostat, so as to present a minute 

 image of the sun. The searching eyepiece 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 eyepiece was traversed towards the 

 disk it became gradually more and more undercorrected. 



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

 equivalent to the effect of the Eoss collar movement upon a minute scale. 



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



* Astronomical Notices. 



