Mr. J. P-. Cooke on a Spectroscope. 



115 



and, the caps having been removed, the image of the slit seen 

 through the observing-telescope was brought into exact coinci- 

 dence with the vertical wire, and the position of the vernier noted. 

 The prism was now brought back to its place by turning the 

 upper plate of the instrument, and the observing-telescope also 

 turned until the position of minimum deviation for the ray I) 

 was attained, and this well-known double line brought to coin- 

 cide with the vertical wire. The limb was then again read ; and 

 the difference of the two readings gave the angle of minimum 

 deviation for the prism. 



In order to show that this method of measurement is perfectly 

 accurate, we give below the angles of minimum deviation of the 

 nine prisms of the Cambridge spectroscope, measured as just de- 

 scribed, and in a parallel column the same angles measured in 

 the old way with the prisms at the centre of the plate. It will 

 be seen that the differences are insignificant, and within the 

 limits of error of observation. 







Measured at 



Measured on 









centre of plate. 



edge of plate. 





No. 



1. 



29 3i 10 



-29° 31 10 





No. 



2. 



29 29 10 



29 29 10 





No. 



3. 



29 28 10 



29 28 10 





No. 



4. 



29 37 



29 36 40 



-20 



No. 



5. 



29 28 30 



29 28 40 



+ 10 



No. 



6. 



29 36 30 



29 36 10 



-20 



No. 



7. 



29 28 10 



29 28 10 





No. 



8. 



29 29 30 



29 29 40 



+ 10 



No. 



9. 



29 28 40 



29 29 40 









267 37 50 



267 37 30 



-20 



Although the adjustments required may appear complicated, 

 they can be made in far less time than it has taken to describe 

 the method. 



It is a well-known fact that when a beam of homogeneous 

 light passes through a prism at the angle of minimum deviation, 

 the incident and emergent pencils make the same angle with the 

 faces of the prism at which they respectively enter and leave the 

 glass. * Hence a second prism, like the first, stands in the same 

 relation to the emerging beam in which the first stands to the 

 incident beam. If, then, after the first prism has been adjusted 

 at the angle of minimum deviation, a second prism be applied 

 against the wheel at the side of the first, by moving the prism 

 slightly to one side or the other it will be easy to find a position 

 in which this prism also is at the angle of minimum deviation, 



