138 Astronomical and Nautical Collections. 



to the middle of the first and that of the second, noting at 

 each the division at which the index stands, together with 

 the number of the entire turns, and by subtracting one of 

 these measures from the other, we shall obviously obtain the 

 required distance. 



I might have begun by describing this mode of observa- 

 tion, but I was afraid of leaving some doubts respecting the 

 simplicity of the causes of the phenomena ; and I thought it 

 better to describe the experiments as they had been made 

 by Grimaldi and by Dr. Young, who received the shadows 

 on a white screen. It is easy to perceive that the mode of 

 observation makes no difference in the results : but it was 

 better to demonstrate, in an incontestable manner, the dif- 

 fraction of the light into the shadow, and the mutual influ- 

 ence of the rays of light on each other ; and I have there- 

 fore deferred the description of the new mode of observation 

 till I came to describe the new experiments that I had made. 



By means, then, of two mirrors, we obtain a very strik- 

 ing example of the effects of the mutual influence of the rays 

 of light, by the reunion of two beams that have been regu- 

 larly reflected at their surfaces. We must not use silvered 

 glasses, but rather glasses blackened at the back, in order 

 that the second reflection may be destroyed ; and metallic 

 mirrors are still preferable. Having placed the two mirrors 

 side by side, so that their edges are in contact, they must be 

 brought very nearly into the same plane, but their faces 

 must be made to incline a very little towards each other, so as 

 to form an extremely obtuse angle. The magnitude of this 

 angle must be regulated according to the interval which 

 separates the images, and this interval must be small, if it 

 is required that the fringes should be broad. But the 

 point which requires the most particular attention is, that 

 the mirrors should not project one beyond the other at the 

 line of contact, for the difference of a few thousandths of 

 an inch might be sufficient to prevent the appearance of the 

 fringes ; and this condition is best fulfilled by the help of 

 the sense of touch, pressing by degrees that which appears 

 to project, against the soft wax by which they are fixed in 

 their places, and observing at the same time, by means of a 



