in the Manufacture and Theory of Gratings. 473 



ruled. Some exact numbers to the millimetre, such as 400, 

 800, 1200, etc., can also be ruled. For the finest definition 

 either 14,438 or 28,876 lines to the inch are recommended — - 

 the first for ordinary use, and the second for examining the 

 extremities of the spectrum. Extremely brilliant gratings 

 have been made with 43,314 lines to the inch; and there is 

 little difficulty in ruling more if desired. The following show 

 some results obtained : — 



Flat grating, 1 inch square, 43,000 lines to the inch. 



Divides the 1474 line in the first spectrum. 

 Flat grating, 2x3 inches, 14,438 lines to the inch, total 

 43,314. Divides 1474 in the first spectrum, the E line 

 (Angstrom 5269'4) in the second, and is good in the 

 fourth and in even the fifth spectrum. 

 Flat grating, 2x3 inches, 1200 lines to 1 millimetre. 

 Shows very many more lines in B and A groups than 

 were ever before seen. 

 Flat grating, 2 x 3£ inches, 14,438 lines to the inch. This 

 has most wonderful brilliancy in one of the first spectra, 

 so that I have seen the Z line, wave-length 8240 (see 

 Abney's map of the ultra-red region), and determined 

 its wave-length roughly, and have seen much further 

 below the A line than the B line is above the A line. 

 The same may be said of the violet end of the spectrum. 

 But such gratings are only obtained by accident. 

 Concave grating, 2x3 inches, 7 feet radius of curvature, 

 4818 lines to the inch. The coincidences of the spectra 

 can be observed to the tenth or twelfth spectrum. 

 Concave grating, 2x3 inches, 14,438 lines to the inch, 

 radius of curvature 8 feet. Divides the 1474 line in the 

 first spectrum, the E line in the second, and is good in 

 the third or fourth. 

 Concave grating, 3x5^ inches, 17 feet radius of curvature, 

 28,876 lines to the inch, and thus nearly 160,000 lines 

 in all. This shows more in the first spectrum than was 

 ever seen before. Divides 1474 Q and E very widely, and 

 shows the stronger component of Angstrom 5275 double. 

 Second spectrum not tried. 

 Concave grating, 4 x 5f inches, 3610 lines to the inch, 

 radius of curvature 5 feet 4 inches. This grating was 

 made for Professor Langley's experiments on the ultra- 

 red portion of the spectrum, and was thus made very 

 bright in the first spectrum. The definition seems to 

 be very fine, notwithstanding the short focus, and divides 

 the 1474 line with ease. But it is difficult to rule so 

 concave a grating, as the diamond marks differently on 

 the different parts of the plate, 



