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



piece of doubly refracting spar a small white disk placed upon 

 a black ground. We may cause the two images partially to 

 overlap each other, and thus have a portion common to both. 

 This portion will be perfectly white ; but the remaining por- 

 tions of each image will evidently have exactly one half of 

 white light. Their tint will therefore agree with that of the 

 revolving wheel when one half of it is painted black. 



But by combining two pieces of spar, images may be form- 

 ed of variable intensity, according to the position of the axes 

 of the spar, and they may either be compared with the gray 

 papers previously graduated, or directly with the indications 

 of the revolving wheel. Having thus found the proportion 

 of white light in the images for any given position of this po- 

 larizing instrument, it may afterwards itself become eminently 

 employed as a photometer. The colours hitherto used were 

 white and black, but in order to verify the results more com- 

 pletely, any two other colours may be chosen, and the mixed 

 tint arising from their union may be employed instead of the 

 gray. 



Having thus considered the appearance of a whirling disk 

 containing one black sector, I next made the following varia- 

 tion in the apparatus. I described a great number of con- 

 centric circles, and drew a common diameter to them, and 

 marked off upon each circumference a number of degrees 

 (reckoned from the diameter) which diminished regularly 

 from 360° to as the circles grow larger ; i. e. in the smallest 

 circle I took nearly all the circumference, in the middle circle 

 I took half the circumference, and in the outer circle the arc 

 was reduced to nothing. The extremities of all the arcs be- 

 ing joined together, formed a line belonging to the class of 

 spirals, but having only one convolution, and bounded by the 

 common diameter. 



(See figure 1.) — This spiral figure was painted black, the 

 rest of the circle remaining white, and the wheel was then made 

 to revolve. The result was a gray surface varying gradually 

 and regularly, in concentric shades, from perfect blackness in 

 the centre to perfect whiteness in the circumference. I then 

 chose a number of coloured papers and cut them into circles 

 of the same size, each of which had this spiral traced upon it, 

 which I afterwards cut out from the rest of the circle. By 

 this means, being all of a size, they could be made to replace 

 each other in a great variety of ways ; for instance, the blue 

 spiral was put on the yellow circle, and the yellow spiral on 

 the blue circle. When in motion, I obtained in each case a 

 surface varying gradually from perfect blue to perfect yellow, 

 the central part being blue in the first case, and yellow in the 



