401- Mr. Talbot's Facts relating to Optical Science. 



be reflected horizontally by a mirror. I will relate a few, out 

 of several experiments which were made in this manner. 



1. About ten or twenty feet from the radiant point, I placed 

 in the path of the ray an equidistant grating* made by Fraun- 

 hofer, with its lines vertical. I then viewed the light whicli 

 had passed through this grating with a lens of considerable 

 magnifying j)ower. The appearance was very curious, being 

 a regular alternation of numerous lines or bands of red and 

 green colour, having their direction parallel to the lines of the 

 grating. On removing the lens a little further from the 

 grating, the bands gradually changed their colours, and be- 

 came alternately blue and yellow. When the lens was a little 

 more removed, the bands again became red and green. And 

 tliis change continued to take place for an indefinite number of 

 times, as the distance between the lens and grating increased. 

 In all cases the bands exhibited two complementary colours. 



It was very curious to observe that though the grating was 

 greatly out of the focus of the lens, yet the iippearance of the 

 bands was perfectly distinct and well defined. 



This however only happens when the radiant point has a 

 Uf/j/swia// apparent diameter, in which case the distance of the 

 lens may be increased even to one or two feet from the grating 

 without much impairing the beauty and distinctness of the 

 coloured bands. So that if the source of light were a mere 

 mathematical point it appears possible that this distance might 

 be increased witliout limit; or that the disturbance in the lu- 

 minous undulations caused by the interposition of the grat- 

 ing, continues indefinitely, and has no tendency to subside of 

 itself. 



2. Another grating was then placed at right angles to the 

 first, and the light transmitted through both was examined by 

 the lens. The appearance now resembled a tissue woven with 

 red and green threads. It seemed exactly as if each colour 

 disappeared alternately behind the other. An alteration in 

 the distance of the lens, altered the tints of the two comple- 

 mentary colours. 



3. A plate of copper pierced with small circular holes of 

 e(]ual diameter and in regular rows, was substituted for the 

 gratings. When this plate was held perpendicular to the ray, 

 it produced a beautiful pattern consisting of rows of circles 

 divided by coloured lines or bars. When the lens was ap- 

 proached to the plate, there was a particular distance between 

 them at which there appeared in the centre of each circle a 



* A plate of glass covered with gold-leaf, on which several hundied 

 parallel lines are cut, in order to transmit the light at eijual intervals. 



