1873.] Circular Solar Spectra. 433 



alluded to at page 428, the microscopic miniature-perspective o£ the room 

 and distant apparatus reappeared ; and after various adjustments I 

 obtained a perfect definition, free from mist and as clear and sharp as 

 that of an opera-glass. The prism and lens of the heliostat then gave a 

 pretty picture of the passing clouds, as well as the small details of the 

 distant objects ; but the instant the sun began to shine, before the rings 

 dazzled the sight, every shining point appeared haloed with a corona of 

 orange and red. I now turned aside the prism : then every polished point 

 in the full sunshine exhibited the same halo. Again in the shade, the 

 picture resumed its sharp definition. "Waiting again for the sun, forth 

 shone the orange haloes. The corrections were diligently plied till the 

 halo nearly disappeared. The sun passed behind a cloud. To my 

 astonishment the former sharp clear prospect was now bedimmed with a 

 general ivhite mist, obscuring all the details before so beautifully clear. The 

 appearance of this white mist, above the best focal point, whenever 

 achromatism was attained by varying the adjustments of the screw 

 collars, now convinced me that the modern English glasses, when 

 rendered achromatic, beget a residuary spherical aberration, obscuring 

 delicate structures (such as I propose to describe further on) by a white 

 mist, corresponding to spherical overcorrection, viz. the condition of 

 the marginal rays for white light cutting the axis at points further from 

 the centre of the lens than the central rays, and that until this fact is 

 acknowledged an insuperable bar to the finest definition will continue to 

 exist. (PI. III. figs. 8&9.) 



Dr. Colonel Woodward, U.S.A., having taken up the research, declares 

 he found it impossible to photograph the most difficult beaded objects 

 unless, upon examining their image on a white screen, he represented 

 the beads red upon a blue ground ; then, using a solution of the 

 ammonio-sulphate of copper to absorb the red rays, and then only, could 

 he photograph the results I had described * (M. Micr. Journ.). 



Eesiduary spherical aberration, it thus appears, is the chief cause of 



* In confirmation of the same principle the late Eev. J. B. Eeade, F.R.S., wrote 1 : — 

 " Dr. Pigott has made also a very decided advance in the better correction of resi- 

 duary aberration, a point which has, I believe, been almost completely ignored (nay 

 even denied), until recently, by accurate observers as well as distinguished opticians. 

 From my own experience in Dr. Pigott's studio, I have no doubt that his colour-test 

 — a most interesting feature in his experiments — is the result of his finer balance of the 

 aberrations . . . This new fact is one of the most striking phenomena in microscopical 

 science of the present day 



" Whether this colour-test is explained on the theory of vibrating wave-length 

 corresponding to the infinitesimal thicknesses of films ... or upon their radiation, 

 refraction, and internal reflection of the spherical beads of which all scales and diatoms 

 appear to be built up, are questions so recondite as to be worthy of the considera- 

 tion of the most advanced physicists of the day." 



As residuary aberration is still denied, I may be permitted to quote ' The Student ' 



1 Popular Science Review, p. 147, No. 35, 1870. 



