Eclielette Grating for tlie Infra-Red . 775 



of one of the screens used in the Joly process of colour- 

 photography. If the grating showed no central image 

 with liglit, it was safe to assume that the metal along the 

 dark line had been forced up and the original surface 

 destroyed. This was usually the case when the grooves 

 were very nearly in contact. If strong central images were 

 exhibited, it indicated that a portion of the original flat 

 surface remained between the grooves. The width of this 

 portion in comparison with that of the roughened portion 

 could be determined, by placing the grating at an angle 

 under the microscope and reflecting light from the linear 

 strips between the grooves. In this way a very perfect 

 knowledge of the exact nature of the ruled surface could be 

 obtained. Another method of studying the surface is to 

 make a cast of it in celluloid or a paraffin composition and 

 section this with a microtome. The optical method gave 

 the best results, however. 



In the majority of cases the crystal was mounted so as to 

 rule a groove one edge of which made an angle of 20° or 

 less with the original surface. With normal incidence this 

 gives us a concentration of energy at an angle of 40°, with 

 practically no energy thrown off from the other edges of 

 the groove, owin^ to the steepness of the angle. This case 

 is shown in fii-. 1 b. The best gratings show no reflexion in 

 the normal direction, i.e. they give no central image. They 

 ^ive, however, a very good reflected image of one's face, 

 when held at an angle of 20°, the image being uncoloured, 

 but slightly diffused by diffraction in a direction perpen- 

 dicular to the grooves. The image is so sharp, however, 

 that the pupil of the eye can be seen without difficulty. 



The gratings behave, with infra-red radiation of wave- 

 lengths, above, say 3 yu,, precisely as an ideally perfect 

 grating, that is they give spectra similar to what we should 

 have with an ordinary grating which threw practically all 

 of the lignt into one or two orders on one side of the 

 central image. 



With visible light their behaviour is most curious and 

 interesting. The central image is usually absent, and we get a 

 blaze of light when the grating is turned at the proper angle. 

 With a symmetrical groove the blaze is seen on both sides, 

 at angles of 45° for normal incidence. This blaze we may 

 term the oblique image. 



If the source of light is white, a lamp flame, for example, 

 the appearance is as shown in fig. 2 d (PL XVII.), which is for 

 a grating with a constant of "05 mm. The position of the 

 central image is indicated by an arrow. It is very faint or 



