AND THE THEORY OF LIGHT. 65 



and may be calculated for each ring or circle on the disc 

 the part within the card being white^ the part without 

 black. Hence the ratio of the part within the card, in 

 any of the above circles, to the part without^ is that of 

 the white to the black. 



98. The number of experiments which can be made with 

 a card of this form is very great and very instructive. It 

 can be made to revolve on its centre^ on one of its angles, 

 on the middle of a side, on the middle of an end^ or on 

 any point removed from the middle of the side or end ; 

 and for every possible change there will be a new figure, 

 having a ring for every distinct radius which can be drawn 

 on the parallelogram from the centre of motion, as well as a 

 modification or change of colour for eveiy change of figure. 



Every possible figure, when made to revolve by hori- 

 zontal motion, takes the form of a disc, and the number 

 of rings on the disc can be calculated on the principle here 

 laid down ; that is, there is a ring for every distinct radius 

 which can be drawn from the centre of motion to some 

 well-defined point, and between two such points there is 

 often a gradation of tint similar to that formed by a 

 penumbra. 



Examining carefully this experiment, one would at first 

 suppose there could be no great difiiculty in determining 

 the numerical value of a ray. The difficulty, however, 

 is greater than it appears at first. The degrees of light 

 are infinite, and the degrees of shade are infinite, and it 

 is, therefore, no easy matter to discover a means of mea- 

 suring their relative proportions. The smallest change, 

 the most minute modification in a diagram, the least 

 alteration in the pitch of the incident light, or in the 

 angle of the diagram to the incident light, an almost 

 inappreciable variation in the motion of the ray created 

 by the machine, each and all tend to produce a problem 

 extremely difficult of solution. 



SER. III. VOL. I. K 



