296 OPTICAL PROJECTION 



find the best arrangement to be as shown in fig. 164, a glass 

 trough such as is used for aquatic animals under the micro- 

 scope, having two thin glass plates A c, B c, cemented with great 

 care, so that there may be no leakage, but that no surplus 

 cement may hinder the fluid filling the ' knife-edges ' at the 

 ends, c, of the end cells. On the outside of the cell over c a 

 stripe must be very carefully blacked, of the exact width that 

 will cover the cemented ends of the glass partitions, so as to 

 leave exposed the extreme * knife-edges ' of the cells, and no 

 more. A slit being placed on the nozzle (the optical front not 

 being used) and a parallel beam- sent through it, is focussed on 

 the screen ; and just where the rays are most concentrated, the 

 cell is placed with the side c to the lens, shading one of the 



end cells with a card, 



C* 



and letting the rays 

 pass through the 

 other up to the edge. 

 The whole cell is 



FIG. 164 filled with alcohol, 



and with a pipette 



are dropped into the end cells, drop by drop, a few drops of 

 saturated solution of fuchsine in the same alcohol. It will 

 be evident that the alcohol cell A c u exactly neutralises the 

 refraction and dispersion of the alcohol in the other cells ; 

 but as the fuchsine is added the colours separate from the 

 slit, red coming between blue and yellow. If the solution 

 is too strong, only red passes : if too dilute, there is no 

 visible dispersion. The use of the double cell is, that if the 

 first be made too saturated, the other may be tried ; and for 

 the same reason it is well so to adjust the glass partitions, 

 as shown in the figure, which is the actual size, that the two 

 may be of somewhat different angles, as 25 and 35. The 

 most brilliant jet should be employed, in order to work 

 through as much as possible of the fuchsine, the effect being 

 chiefly produced at the thinnest part of the cell. Often the 



