192 Further Remarks on [Klil AprTi'Ts^'' 



VI. — Further BemarJcs on High-poiver Definition* By Eoyston- 

 PiGOTT, M.A., M.D., M.E.C.P., F.E.A.S., F.E.M.S., formerly 

 Fellow of St. Peter's College, Cambridge. 



No. II. 



Doubtless there is a new field of observation opening to micro- 

 scopists with the improved magnifying power of modern glasses. 

 There is one point in particular to which I beg to direct the atten- 

 tion of the Society as worthy of closer investigation, for in it lies 

 the germ of great advances in accuracy of definition, — it is the 

 focal image of minute lenses or beading. I arrived at this con- 

 clusion lately from the following observation: — The image for 

 parallel rays lying nearly one-fourth of a diameter outside the 

 surface of a refracting sj)herule, it occurred to me that perhaps it 

 might be possible to form a diatom-lenticular image by selecting 

 beading sufliciently large, and under this impression I now placed 

 a black bar half an inch broad and six inches distant from a 

 diatom, so as to intercept some of the hght reflected from an 

 ordinary mirror. After several trials and searching for a focal 

 point above the surface of the beading, I was rewarded, for the 

 first time with a bar-image, which could be made to gyrate or 

 oscillate according to the movement of the bar ; and subsequently 

 I formed an oval image of the mirror itself, which also moved from 

 side to side in the same direction as the motion of the mirror. 

 This is a perfect and unlooked-for demonstration of the convexittj 

 of the beaded surface. The shadow test of sphericity alluded to in 

 my paper of last year as crescentic, or rather crescent-like, was 

 conclusive ; as only a spherical surface could give symmetrical 

 curvilinear shadows. But the position of the image, without and 

 above, the focus, at once settles the beaded, question. The prophecy 

 might almost be ventured that the time is coming when the images 

 formed by lenticular beads in diatoms will furnish the most exquisite 

 test known for correcting and verifying microscopical definition 

 under high powers. The image was detected with a Wray ^th of 

 excellent quality. 



Already these beautiful objects may be regarded as a mass of 

 highly-refracting spherules.f 



Like a fairy rainbow of infinitesimal drops of solidified silica, 

 their play of colours is remarkable in the extreme. Doubtless, 



* Dr. Pigott wisliea us to state that this article was received by us on the lltli 

 of March.— Ed. M. M.J. 



t I may be allowed to add that I had the good fortune to discover yesterday 

 that the median line of the Formosum is formed of four parallel rows of beads, 

 about one-third the size of the general beading. Every part seems compounded 

 of cohesive spheralcs. 



