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



the distance of the lamp remaining the same, that position of the 

 room is sought at which compensation occurs. In fact, the 

 change in the brightness of the day's light in weather which is 

 not quite clear, or when the sky is uniformly dull, is very con- 

 siderable : this is best seen by directing the illuminating mirror 

 to different parts of the sky, or by comparing the slight intensity 

 of the light reflected from the blue sky with that of a whitish 

 sky. Hence I think this photometer adapted for travellers, whose 

 scientific outfit generally contains a microscope, or, as only a 

 feeble power is required, it can be easily added to it. Measure- 

 ments of the intensity of light while the sun has a high position 

 in the southern hemisphere, compared with those made on the 

 northern hemisphere for the same height of the sun, are entirely 

 wanting. As, by the heating action of direct insolation, it must 

 be considerable, it will also be perceived in the photometric 

 results. 



It will perhaps be convenient to make special photographs 

 for this photometric process. Dickson's monument has pro- 

 bably raised letters on a marble plate, by which the bright- 

 ness of the letters, where the shadow cooperates, is not the 

 same all over the plate, from which the passage in the brighter 

 and darker parts is not simultaneous. This difference is still 

 more perceptible in the raised parts of the edge. The photo- 

 graph of an English bank-note shows the transition more uni- 

 formly. The very fine impression attacked my eyes when 

 viewed too long. I should propose to make a copy of printing 

 of uniform darkness and size, or a simple drawing, for example 

 that of a black cross on a bright ground, or that of a dark ring 

 upon the same ground. Copies of copper-plates are less well 

 fitted, because this negative picture appears objectionably quaint. 

 It will not be difficult to find the most convenient photograph 

 for such a purpose. 



The manufacture of suitable sliders with apertures decreasing 

 in size is also desirable. 



I would here mention a practical application of the method to 

 the estimation of colouring matters, the goodness of which, as 

 in the case of indigo for example, can usually only be ascertained 

 by direct inspection. The impression which such a substance 

 makes upon the eye depends exclusively on the quantity of light 

 which it sends to the eye, and on its colour. It is difficult to 

 make a prismatic analysis with opake bodies, but it is not so 

 with an absorption analysis. Ultramarine appears quite black 

 when viewed through a glass coloured red superficially; by 

 glasses of other colours it appears peculiarly coloured. The 

 quantity of light which it sends through such a glass may be 



