180 Besearehes in Circular Solar Spectra. 



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 heclimmed ivith a general tvhite mist, 

 obscuring all the details he/ore so heautifullij clear. The appear- 

 ance of this white mist, ahove the best focal i^int, whenever achro- 

 matism 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, ob- 

 scuring delicate structures (such as I propose to describe farther 

 on), by a white mist corresponding to spherical over-correction — 

 viz. the condition of the marginal rays for white light cutting the 

 axis at points farther 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. XXXIV., 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, 

 usino- a solution of the ammonio-sulphate of copper to absorb the 

 red rays, and then only, could he photograph the results I had 

 described.* (' Monthly Mic. Journ.') 



Kesiduary spherical aberration, it thus appears, is the chief 

 cause of the difficulty experienced in defining organic particles — 

 such as the molecules of physiological cells, blood-disks, mucous 

 o-lobules, and the discrimination of many forms of disease. It will 

 probably remain uncorrected 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 



* In confirmation of the same principle, the late Kev. J. B. Eeade, F.R.S., 

 wrote ('Popular Science Eeview,' p. 147, No. 35, 1870):— "Dr. Pigott has made 

 also a very decided advance in the better correction of residuary aberration, a point 

 which has, I believe, been almost completely ignored — nay even denied, until 

 recently, by accurate observers ns well as distuiguished 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 micro- 

 scopical science of the present day." . . . 



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

 corresponding to the infinitesimal tliicknesses of films ... or upon tlieir radiation, 

 refraction, and internal retiectiun of tlie spherical beads of which all scales and 

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

 consideration of the most advanced physicists of the day." 



As residuary aberration is still denie<l, I may be permitted to quote ' Tlie 

 Student ' of February, 1870. It states that the writer " has told very plainly two 

 startlin"- and unwelcome truths. First, that observers have not seen their favourite 

 test-ol)icct properly ; and secondly, that theii- best object-glasses are afilicted with 

 sufficient si)licrical aberration to render tlie structure which he describes invisible 

 and that all difficult sec'auj ia iu some suspense through these researches." 



