Besearches in Circular Solar Spectra. 181 



attaining very perfect achromatism, in the microscope, the finest 

 definition will be obtained by stopping out the most obnoxious 

 rays, either by using a monochromatic ray which suits the aplana- 

 tism, or using hluish-green or blue glass * to pale the red rays ; for 

 glasses may be aplanatic to one ray and not to another of a difierent 

 refrangibility. 



Before concluding this part of the paper I may be allowed to 

 make a few practical conclusions for those who may wish to follow 

 up this line of research : — 



1. As stated in the paper " On a Searcher for Aplanatic Images," 

 regarding a convex lens as under-corrected, under-correction is shown 

 by the appearance of the rings below or beyond the focal point and 

 evanishment into mist above it. 



2. Similarity in the rings on both sides (with change of colour 

 also) denotes a balance more or less delicate of the aberrations. 



3. An eccentric position of the solar disk and a crowding 

 of the rings more closely on one side than the other of the circular 

 spectrum denotes parallelism, but non-coincidence of the axes of 

 the convergent and divergent pencils. 



4. Kare and beautiful forms resembling parachutes, vases or 

 comets, made up of ellipsoid, parabolic or hyperbolic diffraction- 

 lines, denote obliquity. (Plate VI., Proceedings K.8., June, 1873.) 



5. Their form depends on the nature of the aberrations pre- 

 sent, and the mode of arranging the axis of the cone of rays forming 

 the solar disk. 



6. Inaccurate centering of the component lenses, either at the 

 heliostat or in the observing or miniature-making objectives, is 

 shown by " eccentric turning " patterns and the appearance of two 

 or several central disks at the smallest focal spectrum. 



7. The ajjparatus necessary to display these brilliant phenomena 

 must be excej^tionally heavy and steady, and the fine adjustment 

 should have a screw 100 threads to the inch; as the ten-thou- 

 sandth of an inch in the axis of observation completely changes the 

 aspect of the phenomena. 



In none of these experiments did the supposed achromatism 

 bear the severe ordeal of the circular solar spectrum. By no 

 arrangements could 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. 



* Appuudix B. 



