Theory of Diffraction- gratings. 91 



grating can be seen on a screen. By the second the aspect of 

 the plane of the grating can be judged of; and when the line of 

 spectra is horizontal, it will be known that the lines of the gra- 

 ting are vertical and parallel to the slit. As object-glass, I am 

 in the habit of using a single lens of about 24 inches solar 

 focus. The eyepiece is a high-power achromatic, supplied by 

 Mr. Browning, and forms, with the object-glass, a telescope 

 of much higher magnifying-power than is ordinarily used in 

 spectroscopes. Without a high power it is impossible to bring 

 out the full value of the grating. In order to obtain the best 

 definition, it is necessary to adjust carefully the aspect of the 

 object-glass ; and I find that the best aspect is not always the 

 same. It is possible that the performance of other optical in- 

 struments might occasionally be improved if means were pro- 

 vided for a slight alteration in the direction of the optic axis of 

 one of the lenses employed. The grating itself I usually place 

 approximately in the position of minimum deviation. 



The copies on worked glass by the ordinary photographic 

 processes and by the modification of the collodio-chloride last 

 described rarely fail in definition. With the original (3000) 

 grating, or with the copies, I can make out nearly, but not 



o ...... 



quite, all that is shown in Angstrom's map. With this grating 

 the third spectrum is generally the most serviceable. When 

 the picked patent plate is employed, there will generally be a 

 proportion whose performance is less satisfactory, though few 

 which would not give very fair results when tested by a low 

 power only. Some cannot be considered inferior to the worked 

 glass, at least when the object-glass is specially adjusted for them. 

 In many cases the definition may be considerably improved by 

 the use of a diaphragm in the form of a horizontal .slit, so placed 

 that only the central parts of the lines of the grating are opera- 

 tive. In respect of brilliancy, gratings may be more quickly 

 judged of; it is sufficient merely to examine the spectra of a 

 candle placed in a dark room. 



The lines themselves are of course too close to be seen with- 

 out a microscope ; but their presence may be detected, and even 

 the interval between them measured, without optical aid, by a 

 method not depending on the production of spectra or requi- 

 ring a knowledge of the wave-length of light. If two photogra- 

 phic copies containing the same number of lines to the inch be 

 placed in contact, film to film, in such a manner that the lines 

 are nearly parallel in the two gratings, a system of parallel 

 bars develops itself, whose direction bisects the external angle 

 between the directions of the original lines, and whose distance in- 

 creases as the angle of inclination diminishes. The cause of the 

 phenomenon will be readily understood by drawing on paper two 



