Absorption Spectra of Dilute Solutions. 123 



glass plate on which, the prisms stand was then placed as nearly as 

 possible horizontal, and the prisms adjusted with their refracting 

 edges vertical by reflecting the light from the slit, c, first from one 

 surface and then from the other into the telescope, K (the slit of this 

 telescope being replaced by an eye-piece with cross threads) nntil the 

 images reflected from the two surfaces were seen in the same position. 

 The telescope, K, was then placed horizontal, and the collimator 

 levelled, so that the images of the slit, c, seen in the telescope were in 

 the centre of the field of view. 



The right-angled prism, E, was then put in position and levelled, 

 so that the light underwent no vertical displacement on passing 

 through it. The Lummer and Brodhun prism is next treated in the 

 same way, and finally the two refracting prisms are put in position 

 and adjusted to minimum deviation. The angle at which the light 

 was internally reflected in the right-angled prism was in my apparatus 

 43 30'. As the critical angle for flint glass is 37 19' there was no 

 danger of the internal reflexion being incomplete. Finally it was 

 found to be of great importance that the light should pass centrally 

 through the tube of solution. To adjust the tube, cross threads were 

 attached to the lens, a. This was conveniently done by means of a 

 brass ring which just slipped over the tube carrying the lens to which 

 the threads were attached. A card with a pin-hole in it was set up 

 at a distance of about a metre and half from the lens in such a posi- 

 tion that the prolongation of the line joining the centre of the slit, 

 c, and the intersection of the cross threads passed through the pin- 

 hole. The tube of solution, also provided with cross threads at each 

 end, was then placed in position and adjusted, so that, on placing the 

 eye behind the little hole in the card, the intersections of all three 

 cross threads fell together. It was found that a perfectly satisfactory 

 adjustment could be got in this way. 



Method of making the Readings. 



The sector being open to its full extent, the brightness of the two 

 lights was arranged by putting a smoke-glass plate in the collimator 

 at d, so that the comparison light was a little brighter than the other. 

 The width ofc the slits was taken as small as was consistent with 

 having sufficient light. Five readings were then made of the ratio 

 between the intensities of the two beams. The tube of solution was 

 then put in position, the two images adjusted to coincide, and five 

 more readings made. Then the readings were repeated, the tube 

 being, however, the second time placed in such a position that the 

 deflection of the light was in the opposite direction ; consequently 

 the tube rotated 180 round its long axis. 



When the means of these sets of readings did not agree, the ad- 



