C. GENERAL PHYSICS. 143 



time between the instant when he thought that this contact 

 had taken place, and that when he was certain that it was 

 past. Bulletin IZebdomadaire, XVI., 21. 



OX THE PHENOMENA OF DIFFPvACTTON PRODUCED BY CIRCU- 

 LAR NET-WORK. 



Soret states that he gives the name of circular net-work to 

 opaque screens pierced by a series of apertures presenting 

 the form of concentric rings. When a beam of light falls 

 upon such a net- work, phenomena of diffraction are produced 

 varying according to the relations which exist between the 

 diameters and the breadths of the rings. Some very remark- 

 able phenomena have been observed by him, which seem to 

 verify the theoretical considerations that he has advanced in 

 reference to this subject. The screens experimented upon 

 by himself were obtained by making a design in china ink 

 in the form of 196 concentric circles, whose radii were pro- 

 portional to the square roots of the natural numbers 1, 2, 3, etc. 

 The first circle had a radius of 25 millimeters. The greatest, 

 consequently, had a radius of 350 millimeters. The rings com- 

 prised between the first and second, between the third and 

 fourth, etc., were blackened. We thus have in white upon 

 black background the figure of a grand positive circular net- 

 work, having 96 concentric rings. This design was repro- 

 duced by photography upon glass, in various scales of from one 

 one-hundredth to one twenty-fifth of the size of the original. 

 A beam of solar light is now introduced into a dark chamber 

 by an opening of any form whatever, and we place a red glass 

 before this opening, thus giving nearly homogeneous light. 

 Then at a proper distance a collimating lens, which renders 

 the rays parallel, and gives at a great distance at the end of 

 a large hall an enlarged image of the opening. Behind the 

 collimating lens we place the circular net-work formed as 

 previously described, the image at the end of the hall con- 

 tinues, only it is a little less clear, and is surrounded by an 

 aureole, which we may attribute to the imperfection of the 

 net-work. If now we place a white screen at the proper dis- 

 tance, we shall obtain a new image of the opening, smaller 

 but distinct. But outside of this image the screen is also il- 

 luminated, as it should be, to accord with the theory. Bring- 

 ing the screen into the second proper position, we observe a 



