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



Intensity of the Light of Optical Instilments. 



The examination of telescopes is very simple. Where, when 

 the telescope is directed upon an infinitely distant object, the 

 rays emerge parallel from the eyepiece, the intensity of the light 

 of the telescope is determined by the brightness of a section of 

 this emergent cylinder of light. The microscope is so placed 

 that, while the aperture of the objective tube covers the aperture 

 of the eyepiece, the axis of the microscope is the rectilinear pro- 

 longation of the axis of the telescope directed towards the heavens. 

 For different instruments which are to be compared with one 

 another, this may be rapidly effected either with a clear or with 

 a uniformly clouded sky. The detenu ination is made by ap- 

 proaching a constant light to the front side of a photographic 

 picture. 



It is not advantageous to use direct sunlight for this illumi- 

 nation of the front side ; for with such strong illumination the 

 writing appears like gold print, which renders the determination 

 more difficult. 



The determination of microscopes takes place in the same 

 manner. 



That of the reflexion of mirrors is effected by the method spoken 

 of when considering the diffusion of rough surfaces. It has surprised 

 me that a carefully polished silver mirror, at any rate under acute 

 incidences, reflects more light upon the glass side than upon the 

 metallic ; in other words, that the addition of the reflexion from 

 the outer surface of the glass more than compensates the loss 

 which the metallic surface coating the glass experiences by the 

 glass. The influence of this external reflexion is determined if 

 the mirror, while at the polarizing angle of glass, is viewed 

 through an eyepiece provided with a polarizing Nicol, so that the 

 reflected light is polarized. The decrease of the intensity in con- 

 sequence of multifold reflexion is easiest obtained by mirrors 

 which can be brought near by means of a screw, where the 

 reflecting side of the one overlaps that of the other laterally. 

 The total reflexion of a rectangular prism is most easily obtained 

 by bringing it under the aperture of the object- tube of the 

 microscope standing almost vertical. The microscope, on the 

 contrary, is almost horizontal in investigating a reflecting prism, 

 or the combination of two, which I have named reversion prism. 

 As this serves to change linearly-polarized into elliptically-polar- 

 ized light, without, as in FresnePs rhombohedron, coming out 

 of the axis of the instrument, the intensity of its light can be 

 compared with that of such a one. The method appears to me 

 important for hollow mirrors and condensing-lenses, from its 

 application to lighthouses with polyzonal lenses. In my resi- 



