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



attained. If the quantity of light incident from below is thereby- 

 more than compensated, the inscription does not appear white, 

 but of a bright subjective colour ; but the transition from dark on 

 light ground, to light on dark ground, is readily perceived. With 

 glass coloured only superficially {Ueberfanggl'dser), an increase of 

 the thickness can only be obtained by superposing them ; with 

 coloured glasses, by means of flat-cut wedges which are pushed 

 over one another ; with coloured mica, by splitting it, and deter- 

 mining the thickness by means of the spherometer. If the com- 

 pensation has been obtained for different-coloured glasses in suc- 

 cession, this serves as a scale of colours of the same brightness. 



To answer the question in what relation the brightness de- 

 creases with increasing thickness, compensation is first effected 

 for the greater thickness, and the magnitude of the aperture 

 diminished by means of the slider until compensation is effected 

 for the less thickness. With parallel incident light, the bright- 

 ness is inversely as the aperture. If the polarizing Nicol is 

 placed in the aperture, the same result can be obtained by turn- 

 ing the analyser ; for this only affects the polarized light incident 

 from below, and not the diffused and therefore unpolarized light 

 incident from above. 



In the same way the relation of the transmitted light with 

 the same thickness of different-coloured substances is obtained 

 by means of the slider or of the Nicol. 



I was much surprised in these experiments at finding how 

 little the eye can judge the intensity of the light which a trans- 

 lucent body transmits in comparison with a transparent one. I 

 constructed a set of glasses out of a cut ground glass, and in- 

 creased their number until the light which they transmitted was 

 equal to that which traversed a red superficially-coloured glass 

 with reflecting surfaces. Both held at the same distance from 

 the eye, or held close before it, appear very different, that is, 

 the red glass much brighter. Unconsciously we connect the 

 idea of distinctness with the estimation of brightness, so that 

 the latter is subordinate to the former. 



The testing of dichroitic crystals is effected by obtaining 

 compensation in different directions. In some which I examined 

 the difference was considerable. The testing of coloured glasses 

 for solar observations, which darken greatly, is effected, after the 

 mirror has been turned aside, by directing the instrument 

 towards the sun immediately after the glasses have been inter- 

 posed. The determination of a still greater darkening, by 

 combining different-coloured glasses, is effected in the same 

 manner. 



The absorption of very transparent substances, as coloured 

 liquids, which must be used in long tubes provided with move- 



