MICROSCOPICAL IMAGERY. 207 



swelled out exceedingly, for a theoretical solar disc of one-millionth 

 of an inch appeared as large as a disc the sixty-thousandth (easily 

 formed by placing a minute lens next the heliostat).* To define 

 accurately bright organic particles, such as those of the smallest 

 disease-germs or the molecules of cancer-cells, is at present unat- 

 tainable. When such delicate forms are in quest, all rays of an 

 aberrating character must necessarily be extinguished. 



In the Podura case noted above, I found the resolution was 

 best accomplished with the i — 50th, when a peculiar blueish- 

 green sunset sky was used to illuminate the beadings ; doubtless, 

 in this case, the diffraction spectra had reached a minimum, 

 whilst the blue light extinguished the aberration of the red rays. 



In my experience, minute organic particles are in general 

 translucent, and give out not only diffractive, but aberrative rays, 

 which conspire to over-power the dehcate rays due to outline only. 



Minute Molecules. 



I particularly wish to inform the readers of this Journal, that a 

 new test-scale, more difficult than usual, has been discovered, 

 obtained from the English Butterfly^ Colias Ccesofiia ; every scale 

 is most minutely beaded with molecules, arranged more or 

 less incurved or straight rouleaus. Some of these I have called 

 coronets. Mr. Powell has received two slides from me, and sees 

 these minute molecules readily with his Apochromatic oil- 

 I — 12th. 



The size of the smallest molecules is about i — 140,000th of 

 an inch. The coronets are composed of beaded rings enclosing 

 a small central dot of light. Had I not noticed similar, though 

 larger structures, these coronets would have escaped my attention. 



First, I had seen plain rings or circular holes in the intercostal 

 spaces of Papilio Mavedo?i. Secondly, dotted rings similarly 

 distributed in Papilio Menmon. Thirdly, rings adorned with one 

 molecule at the circumference can be seen in Paphia Argynnis. 



The test-colias scales are dark, having ribs widely disposed 

 about the i — ioo,oooth of an inch thick. The chaplets of 

 molecules run chiefly in a longitudinal direction, wavy or linear. 



With a fine ^-objective, a minute mottling can be discerned. 



* Described in the last paper. 



