OCTOBEE 26, 1900.] 



SCIENCE. 



62£ 



over, s/a = i//x-, so that the strong image is 

 usually remote. The projected grating is 

 here taken as the larger, a>6. If a<h 

 the corresponding image space will be 

 s' =6(1 + (^xj'x + y)(2a — b/b). 



In Fig. 7 the ratio a/b is 3/2. Table 3 

 shows the focal planes /? and p. to be pro- 

 nounced. The magnification at /^ is 2, with 

 strong brown lines on a white ground which 

 contains faint traces of a pinkish diffrac- 

 tion band in the middle. 



the light areas which are alternately white 

 and colored reddish. 



The final case to be exhibited in detail 

 is Fig. 8, where a/b = 4/3. The focal 

 planes e and ? are marked phenomena, the 

 latter at tlie long distance of 6 meters from 

 jB, strong and coarse as usual. With the 

 mica screen clean cut dark bands .2 cm. 

 broad and .7 cm. apart, cover an area of a 

 square foot. If B is the projecting and A 

 the projected grating {a/b = 3/4, light re- 



If the ratio a/b is 2/3, A' and B' may 

 represent the positions of the grating, the 

 light retrogressing, so that S' is the corre- 

 sponding focal plane. It is marked // in 

 Table 4, where raoreover p. is again repro- 

 duced as the second focal plane of this 

 series. The coarse images for this and 

 succeeding cases of long distance (6-10, 

 even 30 meters) , are a striking feature. The 

 phenomenon becomes fainter but otherwise 

 more remarkable and much larger if the 

 ground glass or, better, the mica screen is 

 placed before the first grating. The dif- 

 fraction character then becomes manifest in 



trogressing), Table 4 shows the focal plane 

 at 4 meters well marked, and found from 

 Fig. S by prolonging the lines backwards, 

 in the direction BA. 



The other cases of Tables 3 and 4 are 

 found by subjecting Figs. 7 and 8 to a 

 homogeneous strain, with the principal 

 strains in the horizontal and vertical di- 

 rections. Similarly, 7 would follow from 8 

 or from the above figures. Focal planes 

 corresponding to S are usually well shown. 



As a rule, therefore, the diagrams are a 

 convenient means of predicting the rela- 

 tions of size and distance of the images. 



