266 



Dr. G. W. Royston-Pigott. 



research, is beset with, many difficulties and variable factors, some of 

 which may easily be overlooked or even unsuspected. 



As nearly all organic tissues teem more or less with, minute mole- 

 cules, their variable behaviour and optical appearances have received 

 close attention for many years, and I have concluded that — 



A. — A refracting molecule changes its appearance and phenomena 

 according to the nature of the fluid by which it is surrounded, also 

 with the fluid in which the objective may be immersed.* 



B. — It changes also, in an extraordinary manner, according to the 

 angular aperture of the objective employed, according to the refrac- 

 tioDS of the media, and the direction of illumination and the paral- 

 lelism or degree of convergence or divergence of the illuminating 

 pencil transmitted. 



C. — Other interesting factors are the residuary aberrations of the 

 compounding lenses of observation and the greater or less intensity of 

 diffraction phenomena introduced by the mode of illumination. 



In a short paper, it will be convenient partly to deal with these 

 questions as they arise in the experiments rather than seriatim. 



(2.) Excessive Angular Aperture considered. 



The principal and chief, I may say the most valuable, feature in 

 the appearance of a refracting molecule is the extraordinary variability 

 of the blackness and thickness of the marginal annulus. 



This thickness and consequent visibility is dependent on the value 

 of the refraction into the given media, and the angular aperture of 

 microscopic observation : partially also upon the situation of the focal 

 point of the lenticular illumination. 



'Example 1.— A glass spherule (0""1 diameter) is examined with a 

 pocket lens. An intensely black broad ring is seen against the light. 

 The same black ring is visible in bubbles frequenting plate glass in 

 windows. The angular aperture of the pupil of the eye is about a 

 degree for an image seen at 10 inches distance. Melted glass fila- 

 ments are also instructive (see figures a, /3, 7, B, e, rj, 0). 



* It is not many years since lines on diatoms were all that were searched for. 

 But as these are formed by an aggregation of siliceous spherules, they present extra- 

 ordinary opportunities for investigating optical characters which must necessarily 

 belong to them — of a degree of minuteness of a highly satisfactory order — such as 

 black annuli, focal points varying with chromatic refraction, shadows varying in 

 contour according to the extinction of transmitted rays by obliquity of illumination : 

 appearances changing with the refractive index of the media in which they are 

 immersed. Only the very finest glasses extant can display the black margin of 

 a diatomic spherule the 40 ooo of an inch in diameter ; and very few, if any, can be 

 obtained capable of displaying focal points and the natural coloured foci at different 

 chromatic foci. 



