158 Transactions of the Societij. 



which the third view (as to all-round vision and dissimilar images) 

 has been founded, and for this purpose it will be necessary to refer 

 to the paper by Dr. Koyston-Pigott, F.R.S., in which the subject is 

 dealt with.* 



After reminding his readers that he had shown that spider- 

 lines, miniatured down to the fourteenth part of the hundred- 

 thousandth of an inch, were distinctly visible to ordinary good 

 eye-sight under proper microscopical manipulation (an experiment 

 which, I may remark in passing, has not a satisfactory foundation), 

 Dr. Pigott says : — " Under these circumstances it was interesting 

 to know whether real objects could be detected by the Microscope 

 in the surprising degree of attenuation represented by the mil- 

 lionth." Minute particles of mercury were obtained by smashing 

 some with a watch-spring, and they were mounted in petroleum 

 under a thin cover. A vertical illuminator was used to converge 

 rays downwards, through the objective, upon the preparation. In 

 a darkened room minute disks became visible, and upon some of 

 them clusters of minute black points were seen with a power of 1000 

 diameters. Comparing them with a micrometer spider-line rc^oir 

 inch diameter, some of the points were found to be decidedly 

 smaller. Under 1000 diameters the particle was magnified one 

 hundred times in the micrometric focus, and then appeared less 

 than the spider-line. Its real diameter was therefore less than 

 T^o of To ¥0 0- iiich, or less than the millionth of an inch, and 

 the writer draws the conclusion that " real objects of unsuspected 

 minuteness may be microscopically displayed as well as minute 

 miniature images." To this part of Dr. Pigott's observations it 

 may be pointed out that it has never been supposed, so far as I am 

 aware, that there is any limit of visibility in the Microscope other 

 than that imposed by the sensibihty of the observer's retina, the 

 correction of the objective, and the illumination. The question of 

 a limit of visibility is quite distinct from that of a limit of separation, 

 just as in telescopic vision a single star is always visible, however 

 small its visual angle, provided it is sufficiently bright, but a double- 

 star requires a certain minimum aperture of the objective, dependent 

 on the angular distance of both stars. 



Discussing the variability of the blackness and thickness of the 

 marginal annulus of refracting molecules, as exemplified in a glass 

 spherule • 1 inch diameter, and in the featherlets of the death's- 

 head moth and plumelets of Hipparchus Jaidra with objectives of 

 20° Ang. Ap. power 200, and 140° Ang. Ap. power 800, he 

 writes : — " If then the minute fibrillae of the plume can be clearly 

 distinguished as closely packed black lines at a visual angle of 

 20 seconds with a low aperture of 20°, this result is fatally opposed 

 to the popular idea that very close lines, or very minute lines or 

 bodies, can only be distinguished with large angular aperture. 



* Proc. Roy. Soc, xxxi. (1881) pp. 260-78. 



