M. Babinet on the Law of the Colours, ^c. 109 



^RT. XVI. — On the Law of the Colours seen by transmission 

 through Grooved Surfaces. By M. Babinet. 



The paper by M. Babinet, of which we propose to lay before 

 our optical readers the most important part, was read to the 

 Philamothic Society on the 8th December 1827, and has been 

 published in the Ann. de Chimie for February 1829. The 

 phenomena of which our author investigates the law, ar^ 

 those which have been so accurately measured and described 

 by Fraunhofer, and which were made by transmitting the light 

 of a narrow aperture through systems of equidistant parallel 

 wires of very small diameter, or through systems of grooves 

 made upon glass by a diamond point. As these phenomena 

 are already well known to the readers of this Journal, we shall 

 proceed to M. Babinet's explanation of them. 



" To conceive this law, and to give an explanation of it, let 

 us suppose that MN, Plate II. Fig. 1, represents this system 

 of grooves of which LP, QA, KB, RN, are the full or opaque 

 parts not permeable to light, and HL, PQ, AK, BR, the 

 transparent parts. The phenomena depend on the width of 

 the equal intervals HP, PA, AB, BN, composed of one opaque 

 and one transparent part. Let us take one of these inter- 

 vals AB, so situated that to the eye placed at C, the differ- 

 ence of the lines BC and AC may be equal to the length of an 

 entire undulation for one kind of light. The incident rays 

 SH, SA, SB, &c. being perpendicular to the plane of the 

 j plate MN, and radiating from a point sufficiently distant, 

 I and the lines AC and BC being sensibly parallel on account 

 I of the extreme smallness of AB, the arc AG described round 

 ! C as a centre (so that BG = BC — AC = X) may be considered 

 I as a perpendicular common to the lines AC and BC, and BG 

 1 equal to X will express the retardation of a ray which follows 

 the line SBC, compared with a ray which follows the line SAC. 

 Let us suppose for a moment the interval AB to be quite open, 

 and through I, the middle of AB, let us draw SIC, the retar- 

 dation of which will consequently be one-half that of BG, that 

 is a semi-undulation in relation to the ray SAC. On this 

 supposition, it is obvious that the ray which goes from A to C, 



