144 TEAR-BOOK OF FACTS. 



which he walked, the snow-clad country seemed to assume other 

 colours. Sir D. Brewster referred to a paper which appeared in the 

 Philosophical Magazine many years since, where colours such as 

 those on the diagram were described as manifesting themselves 

 under somewhat similar circumstances. The opinion of the members 

 of the Section who joined in the conversation seemed to be univer- 

 sally, that the effect was due to the power of the retina to recover 

 the power of uoticing the several colours with different degrees of 

 rapidity. 



THE PSEUDO-DIASCOrE. 



By means of this instrument an aperture transmitting light is 

 made to produce on one eye an isolated impression, while the other 

 eye is directed to an opaque body, such as the hand held before it. 

 The image of the aperture is then found to be transposed, audits 

 perception ceases to be assigned to the eye by which it is really seen, 

 — the effect being that a perforation appears in the opaque body, 

 through which the light seems to shine upon the eye by which this 

 is viewed. The principle illustrated by this instrument, according 

 to the author's view, is the essentially gouiometrical and deductive 

 nature of the visual act, whenever the distances of bodies are per- 

 ceived, and their relative positions in space assigned. — F. 0. Ward ; 

 Proc. Lit. and Phil. Soc. Manchester. 



TO MEASURE DISTANCES. 



Mb. Patrick Adie has described to the British Association an 

 Instrument for Measuring Actual Distances. Prof. Stevelly stated 

 that the Master of Trinity College, ('ami nidge, had, at the South- 

 ampton meeting of the British Association, explained a method 

 similar in principle to this for observing the heights of the clouds. 

 But from the difficulty experienced in getting a sheet of water for 

 a reflector, both calm and at a sufficient distance below the observer 

 to ensure sufficient accuracy, he (Prof. Stevelly) had bee!) led to a 

 modification of Dr. WhewelT s method, by using mirrors in away 

 almost exactly the same as that of Mr. Adie ; but noi ;:• at in 



arrangement, nor admitting of such acoiracy of observation as his 

 instrument, which he hailed as affording, among many other 

 not only a means of observing the heights of clouds of different 

 modifications, but also the distance from the observer of those lying 

 off towards the horizon, a fact very difficult, under many circom* 

 stances, hitherto to determine, or even to estimate correctly. 



SPECIFIC GRAVITY OF Till: DIAMOND. 



Tin - , specific gravity of this gem is generally stated in eli mentarj 

 works to range from 8*5 to 8*65 ; but these numbers do not repre- 



an of recorded experiments, as will be seen by the fol- 

 lowing table : — 



