272 



Dr. G. W. Royston-Pigott. 



Result. — Many V-shaped rows are visible : double molecules here 

 and there cross between the ribs. Rapid bat delicate changes of 

 pressure cause minute oil globules to flash up and down with focal 

 points. 



An interesting demonstration of the molecules completely filling 

 up the scale, may be given by rotating the object so as to receive 

 oblique pencils successively. The black crescentic shadows of the 

 molecules may then be seen in different positions relative to the ribs 

 of the scale (fig. 6). Fig. 10. 



I must confess that in my earlier attempts to account for the 

 extreme and often insurmountable difficulty attending the demonstra- 

 tion of minute crowded molecules in many translucent organic forms, 

 it was erroneously concluded that residuary aberration was the only 

 cause of their obstinate obscuration. I am now able to amend this 

 conclusion by adding another obstacle, excessive angular aperture, 

 which I found attenuated margin. 



Indeed, upon patiently reducing the residuary errors to a minimum, 

 by means of supplementary lenses, as described in the Transactions 

 of the Royal Society,* it was further discovered that the mode there 

 described very considerably reduced also angular aperture. This 

 result necessarily followed the very considerable shortening of the 

 posterior foci of the objectives employed : at the same time, not- 

 withstanding the full aperture of the lenses is engaged, the residuary 

 aberration of the whole is reduced also, in consequence of angular 

 aperture being at the same time reduced : this large aperture being 

 indeed the great source of spherical error. 



The angular aperture may be reduced in the same objective by 

 introducing stops between the back sets of lenses. It is better, how- 

 ever, to have a series of objectives of excellent quality of graduated 

 apertures. Aperture can always be reduced by shortening the 

 eye-tube, or by using low power objectives and deeper eye-pieces to 

 get the same power. It may also be reduced in a manner by using 

 an illuminating cone of light, whose aperture can be diminished at 

 will. 



There is, it may be said, something unnatural in the mode of vision 

 intrinsic to very high angled glasses. It is undoubtedly true that 

 such a glass presents an all-round vision. It really conveys visual 

 rays from a given brilliant particle, at every inclination in azimuth 

 and altitude ; and this too at one and the same instant. 



To illustrate this position, a minute die may be imagined the hundred 

 thousandth broad. The highest angled objective really enables the 



* "Phil. Trans.," vol. 160, 1870. "On a Searcher for Aplanatic Images," by the 

 Author. 



