434 



Br. Royston-Pigott on 



[June 19, 



the difficulty experienced in defining organic particles — such as the 

 molecules of physiological cells, blood-disks, mucous globules, and the 

 discrimination of many forms of disease. It will probably remain un- 

 corrected until opticians and observers abandon the false standards of 

 definition still in vogue. If, then, it is at present impossible to avoid a 

 residuary spherical aberration, whilst 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 

 aplanatism, or using bluish-green or blue glass * fe pale the red rays ; 

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

 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," re- 

 garding a convex lens as undercorrected, undercorrection is shown by 

 the appearance of the rings below or beyond the focal point and evanish- 

 ment 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 excentric 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 (figs. 28 & 29). 



4. Bare and beautiful forms, resembling parachutes, vases, or comets, 

 made up of ellipsoid, parabolic, or hyperbolic di&action-lines, denote 

 obliquity (figs. 16-34). 



5. Their form depends on the nature of the aberrations present, 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 " excentric 

 turning " patterns and the appearance of two or several central disks at the 

 smallest focal spectrum. 



7. The apparatus necessary to display these brilliant phenomena must 

 be exceptionally heavy and steady, and the fine adjustment should have 

 a screw 100 threads to the inch, as the ten thousandth 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 



of February 1870. It states that the writer " has told very plainly two startling and un- 

 welcome truths. First, that observers have not seen their favourite test-object properly ; 

 and, secondly, that their best object-glasses are afflicted with sufficient spherical aberra- 

 tion to render the structure which he describes invisible. . . and that all difficult- seeing 

 is in some suspense through these researches." * Appendix B. 



