Microscopical Researches in Hujli Power Definition. 21 3 



observer to collect rays emanating from four sides, and the top at the 

 same instant. The human eye could at most view three sides at once. 

 Doubtless, the effect of this angular vision all round the corners, 

 causes particles to look spherical when sufficiently minute, even if 

 cubical. 



To these considerations we may add the complete attenuation of 

 annuli (which are black in refracting molecules with low angular 

 aperture), and the comparative invisibility of masses of molecules. 



It may be remarked here, that the resolutions described have chiefly 

 been accomplished by placing the objects in a condition to develope 

 black annuli or crescentic shadows, and chiefly by reducing the 

 aperture. It is also remarkable that the popular markings of the 

 Podura scale lie in a higher plane than the molecules, and having 

 been sought hitherto in a wrong focal plane, were naturally missed. 



In cases, however, where the composite spherules are much more 

 crowded and much more minute, mere reduction of aperture is 

 inadequate to discover their existence. Their excessive delicacy of 

 form and translucency of substance, the difficulties introduced by 

 unmodified diffractions, and by searching the wrong focal planes, mis- 

 management of shadow-phenomena and uncorrected residuary aberra- 

 tion combine together to present such formidable obstacles to success, 

 that few can congratulate themselves on the complete resolution of 

 this celebrated test object. 



[It having been pointed out that the apparent headings on gnat 

 scales have been degraded by the application of the term spurious, it 

 may be said in reply that no proper explanation of these appearances 

 has been given. On the other hand, spherules much finer may be dis- 

 tinguished on several delicate scales. 



Mr. De La Rue pointed out many years since the beaded character 

 of insect striated scales ;* Mr. Stephenson has minutely described 

 those of the gnat scale ; and Mr. Slack other beaded scales, in 

 " Student," vol. v. 



In order to place the matter beyond reasonable doubt, and to clear 

 up the structure, diligent search has been made among a considerable 

 variety of these objects, and in the 2nd Plate drawings are given from 



* " Trans. Mic. Society," vol. iii, December 20th, 1848. In this paper Mr. De La 

 Rue was able to show that with a l-12th Ross, the ribs of the butterfly scales of 

 AmatJiusia Rorsfieldii, were from the l-6,000th to 1 -10,000th apart centre to centre, 

 and that cross striae, under a power of 850, showed protuberances on beaded lines 

 and the latter focussed at their summits appeared as brown dots. The scale viewed 

 from the under side exhibited the lower membrane as slightly undulating, probably 

 from being dry, p. 36. He proceeds : " Some of my friends thought the constricted 

 appearance of the cross striae is due to overlaying pigment cell : this, in my opinion, 

 is not correct, as I have convinced myself by repeated examinations, more especially 

 from the under side, that the striae themselves are really beaded." 



