722 



SUMMABY OF CURRENT RESEARCHES RELATING TO 



being represented by the shaded space in the fig. In other words, 

 the total quantity of light which in air is thrown upon the element a b, 

 is by means of the hemisphere condensed upon the smaller element a yS, 

 so that the hemisphere will admit to the element ah, wider pencils 

 from the points O P than are admitted to it in air. Though the 



Fig. 175. 



angle c P is the same in both cases, the quantity of light con- 

 veyed within this angle to one and the same surface element is 

 greater in glass than in air. 



As to the measure of the increase of light, it may be shown that 



— r- = — ; i. e., if w = 1 • 5 (the refractive index of glass), — - = — 

 a 6 « ' ' ^ ^ ® ^' a 6 3 ' 



or a 6 is half as large again as a ,6. The increase of light is there- 

 fore as 9 : 4, or 2^ times (n^). This is in agreement with the expres- 

 sion for the numerical aperture in the cases of air and glass, which, 

 for the same angles, are always as 2 : 3.* 



Discovery of Pseudoscopy-.j — Under the title of the " Discoverer of 

 a Singular Optical Illusion," Prof. Govi says, " Of all optical illusions, 

 that is certainly not one of the least remarkable by which, in looking 

 at objects in slight relief or slightly depressed (as coins, seals, &c.) 

 with a compound Microscope or a telescope which reverses the ima^e, 



* See also on this subject this Journal, i. (1881) p. 329. 

 !' t Atti R. Accad. Lincei— Transunti, vii. (1883) pp. 183-8. 



