Mr. H. F. Talbot's Experiments on Light. 331 



latter. But the most curious part of this experiment was to 

 observe the neutral tint, through which the colour passed at a 

 certain point. When this neutral tint offered any singularity 

 that made it desirable to examine it more particularly, it was 

 easy, by measuring its 

 distance from the cen- 

 tre, to compute how 

 much of each colour 

 entered into its compo- 

 sition ; for instance, if 

 the diameter of the 

 wheel, formed of yel- 

 low and blue, was six 

 inches, and the tint in 

 question was two inches 

 from the centre (sup- 

 posed blue), it contained 

 two thirds of blue and 

 one third of yellow, and 

 therefore I had only to 

 make a blue circle, and 

 place on it a yellow sector of 120°, to obtain infallibly the 

 same tint over all its surface. 



It may be objected to these experiments with revolving disks, 

 that they afford no assistance when the light of lamps, &c, is 

 the subject of comparison. But in that case, the principle re- 

 maining the same, the application of it must be varied, and 

 we must observe the luminary through a revolving wheel 

 whose spokes are gradually tapering towards the centre. 

 If there are twelve spokes each of which forms a sector of 

 five degrees, their sum is 60 degrees, and the proportion of 

 this to the whole circle, viz. one sixth, is the degree of obscu- 

 ration produced by the wheel when rapidly revolving upon all 

 objects that are seen through it. 



If the wheel has eighteen spokes, each of which is a sector 

 of 10°, their sum is 180°, and therefore, a lamp seen through 

 them loses one half of its light, and every other luminous ob- 

 ject the same constant proportion. But since it is requisite 

 to have the power of producing a variable obscuration, this 

 may be accomplished by having a second wheel similar to the 

 first, and placed close to it upon the same axis, so that they 

 may be capable of being fastened together in any required 

 position. If they are placed so that their spokes correspond, 

 the two wheels will, of course, only produce the same effect 

 that either of them would separately ; but if the spokes of one 

 correspond to the intervals of the other, they will prevent any 



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