334 SIR AV. DE W. ABNEY : MODIFIED APPARATUS FOR MEASUREMENT OF COLOUR 



silvered mirror, inclined at about 45 to the axis of the lens. The reflected beam is 

 again reflected so as to pursue a course roughly parallel to the main spectrum, so that 

 two similar spectra are placed side by side. The accompanying diagram will show 

 the arrangement. 



55 



B 

 \ 



Diagram of mo'litiud apparatus for the colour patch. 



As in the apparatus described in " ( 'olour Photometry," Part 111. (' Phil. Trans.,' A, 

 L892), K is the soui'ce of light used outside a darkened room, LI, L> are lenses 

 throwing an image of the source of light on the slit S, of the collimator C. TJie 

 parallel beam passes through the prisms I 1 ,, P., and is received on a colour-corrected 

 photographic lens, L h of sufficient diameter to take in the whole of the light coming 

 through the prisms. 



The lens forms a spectrum on a focussing screen at D,, which can be removed and 

 slits S 2 placed in the image. L (i collects the colours and gives an image of the face of 

 the prism P, on the screen B. When slits are placed at D u the image is of the mixed 

 colours passing through them. 



Behind the lens L, is placed a semi-silvered mirror M ]; reflecting, as nearly as 

 may be, the same amount of light as is transmitted through it. If the mirror be on 

 a plate of glass with parallel sides, it should be as thin as possible, to avoid any 

 serious mixture of colour in the second spectrum clue to 

 the reflection of the unsilvered surface. If a plate be made 

 up of two thin prisms, as in margin, with the surface AB 

 of one of them silvered, the transmitted beam is not deviated, and the beams reflected 

 from DB and AC are diverted and not used. 



