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Prof. Stokes on an accurate Method 



[June 20, 



The mean reading "for achromatism by itself alone gives nothing, 

 and mnst be combined with another determination in order to be avail- 

 able. The second determination which I fonnd it most convenient to 

 make was that of the angle of incidence of a particular part of the 

 spectrum on the second prism for a known azimuth of this prism. 



If the brightest part of the spectrum were marked by a definite line, 

 we should choose that. As it is, the line D, though not exactly at the 

 brightest part, lies sufficiently near it for our purpose, as will be better 

 understood presently. No dark lines can be seen in the reflected light, 

 because we are using an aperture, and not a mere slit. But if the slit 

 be illuminated by a soda flame instead of daylight, a well-defined 

 yellow image of the aperture will be seen, the fiducial edge of which 

 can be pointed at with precision. 



To determine the angle of incidence I have employed two methods, 

 both of which I think it will be worth while to describe. The first is 

 the less direct, and involves a little subsequent calculation, but has the 

 advantage of not requiring any additional apparatus beyond what is 

 wanted for the determination of the azimuth of achromatism. In 

 carrying out the first, and in applying either, I suppose the angle of 

 the prism and its index for the line D known from a determination in 

 the usual way. 



First Method. — The azimuth of minimum deviation for the line D 

 could be determined pretty fairly by bringing the wire of the viewing 

 telescope to the fiducial edge when the aperture is illuminated by a 

 soda flame, and the edge is in or near its stationary position, and taking 

 the mean of several determinations of the azimuth at which the edge 

 is stationary. The angle of incidence for minimum deviation is vir- 

 tually measured in the process of finding the index, and is therefore 

 known ; and by applying with its proper sign the difference of mean 

 readings for achromatism with daylight and for minimum deviation 

 with a soda flame, we get the angle of incidence for achromatism. 



This method is mentioned merely as naturally leading up to that 

 actually employed. It must be rejected as too slovenly, since the 

 uncertainty of the determination of the azimuth of minimum deviation 

 is liable to be greater than that of the azimuth of achromatism. 



If we place the wire of the viewing telescope some way off the sta- 

 tionary position of the fiducial edge, there are two azimuths of the 

 prism at which the edge will be on the wire, across which it will move 

 with a finite velocity as the prism is moved in azimuth. Hence either 

 azimuth could be determined with accuracy, and if they were equi- 

 distant from the azimuth of minimum deviation, the latter could be 

 determined at once. This, however, is not the case, but nevertheless 

 the azimuth of minimum deviation can be determined from the result. 



Since the course of light may be reversed in refraction, it readily 

 follows that in passing from one to the other of two azimuths which 



