16 Prof. H. W. Dove on a New Photometer. 



fifty times. The illumination from below gives a deep black im- 

 pression on a white ground ; and if the mirror is covered, illumi- 

 nation from above gives a white impression upon a dark ground. 

 From this it would appear probable that the printing would dis- 

 appear if the light incident from above and from below had the 

 same intensity, or if there were a definite relation between them, 

 inasmuch as the angle under which they are incident may be 

 different. If in the objective tube a polarizing Nicol be fitted, 

 and if the ordinary ocular be replaced by that which contains an 

 analysing Nicol, the inscription disappears on turning the ana- 

 lysing arrangement. The smallest further rotation transforms 

 the previously dark inscription into a white one, — a proof of the 

 great delicacy of the method. This is further seen in the fact, 

 that if, at the position at which the inscription disappears, a 

 feebly dim glass be interposed, the white inscription immediately 

 appears on a dark ground if it be interposed in the light incident 

 from below, while the bright inscription appears on a dark ground 

 if the glass is interposed in the light incident from above. 



If the light incident from below be twice successively dim- 

 med, so that, from the disappearance of the inscription, it coun- 

 terbalances the light which is incident from above v/ith unaltered 

 brightness, it is at once clear that the quantities of light must in 

 both cases be the same, inasmuch as the rays fall on the micro- 

 scopic object under exactly the same conditions. If now the 

 methods which, with sources of light of different brightness, 

 require the deadening of the stronger so that both shall be 

 equal, contain also in themselves a determination of the 

 degree of this deadening, a quantitative determination of their 

 different intensity under the same conditions follows directly 

 therefrom. 



In modern microscopes there is the arrangement that the illu- 

 minating mirror can be placed aside by a double angular motion ; 

 further, that the instrument itself can be removed from the ver- 

 tical into any other position, which, as it only deviates consider- 

 ably from the horizontal in certain special cases, I shall designate 

 the horizontal, in opposition to the vertical position, in which the 

 use of the mirror is presupposed. 



The modes of deadening are as follows : — 



1. Diminution of the aperture of the objective tube. 



2. Removal of the source of light from the same. 



3. Increase of the acting surface of the source of light by 

 inclining it towards the aperture, which represents the rectan- 

 gular projection of that surface ; in which case the cylindrical 

 aperture can be so arranged, by adding a tube blackened on the 

 inside, that only parallel rays fall on the photographic picture. 



4. Rotation of an ocular provided with an analysing Nicol, 



