488 



Prof. Stokes on an accurate Method 



[June 20, 



troduce in putting it in practice, I find it to be no less excellent than 

 easy. 



The experimental arrangements are a good deal simplified by making 

 the prisms to be compared achromatize successively one and the same 

 prism chosen arbitrarily, and retained in a fixed position, instead of 

 making them directly achromatize each other. I will first describe 

 the method as I used it, premising that as the prisms for which it was 

 primarily designed were not of good glass, I was content with less 

 perfect arrangements than would have been desirable for really good 

 glasses. Nevertheless the apparatus when used with good prisms 

 gave very good results. The object observed was a vertical slit, so 

 wide that when viewed through the opposed prisms a broad white 

 stripe was seen, merely fringed at the two edges with the secondary 

 green and purple. The slit was fixed at one end of a horizontal plank, 

 near the other end of which was fixed a vertical lens of about four 

 feet focus. The slit was in the principal focus of the lens. The 

 plank rested on three knobs, one under the slit, the other two near 

 the other end. This mode of support prevented torsion of the plank, 

 which would have produced a lateral derangement of the slit. The 

 rays from the slit, rendered parallel by the lens, fell upon a prism, 

 which I will call the primary prism. This prism is fixed during the 

 observation. Its angle and dispersive power, and the azimuth in which 

 it is set, are arbitrary within wide limits. 



The beam emerging from the primary prism falls, at a little distance, 

 on a second prism resting on a stand movable along with the verniers 

 of a protractor reading to minutes (for the loan of which I am in- 

 debted to Professor Miller), which latter rests on the plank. As a 

 matter of convenience, the prism has an independent motion in azi- 

 muth, but when once placed in a convenient position it is not after- 

 wards moved except along with the verniers. 



The beam refracted and dispersed in contrary directions by the two 

 prisms is received on the object-glass of an achromatic telescope un- 

 connected with the graduation, with which the image of the slit, if so 

 wide an aperture may so be called, is viewed. In general the edges of 

 the aperture are seen coloured blue and red, which colours, on turning 

 the prism through its azimuth of achromatism, are changed into red 

 and blue by passing through the secondary green and purple. That 

 edge of the slit at which the green is seen is alone attended to, and is 

 treated as the fiducial edge. The prisms are deemed to achromatize 

 each other when this colour is midway between yellow and green. 



Should the observer wish to aid his judgment by observing the 

 purple as well, it would be proper to use an aperture of the form 

 represented in the figure, so that the green and purple should be seen 

 right and left along a common edge, and therefore at the same angle 

 of incidence, or rather at angles of incidence having the same hori- 



