176 



Dr. J. C. Bose, On the Determination of the 



radiator, levels the grating till the image of the eye is seen reflected 

 by the mirror. 



I first obtained an approximate value of the wave-length with a 

 2-cm. grating, and then took careful and systematic readings with the 

 different gratings. By different gratings is meant the same carved 

 piece of ebonite, on which strips of different breadths were successively 

 applied. The grating was found fairly adjusted, and the readings 

 taken on the right side of the grating agreed well with the corre- 

 sponding ones on the left side, I did not, therefore, think it necessary 

 to take double readings, but took the various readings alternately on 

 the right and on the left side. In one case only I found the grating 

 on one side giving slightly better reading than the other. When the 

 incident angle is too oblique, the diffracted image is not sharp, and I 

 therefore did not extend the reading beyond 40 of incidence. Spectra 

 of the tirst order only were observed. The response in the receiving 

 circuit was somewhat feeble when 1 cm. or 1*5 cm. grating was used. 

 But a 2-cm. grating gave stronger indications. With 2*5 and 3 cm. 

 gratings the response was very energetic and the definition of the 

 diffracted spectrum very sharp. For example, when the receiver was 

 kept fixed, and the angle of incidence gradually varied, there was an 

 abrupt and strong response produced in the receiving circuit, as soon 

 as the angle of incidence attained the proper value. A slight varia- 

 tion of this angle, even of less than a quarter of a degree, produced 

 displacement of the diffracted image, and there was then no further 

 action on the receiver. Had my graduated circle permitted it, I 

 could have got more accurate readings. The radial arms carrying 

 the receiver and radiator were of too primitive a design to make it 

 worth while to attempt greater accuracy. I give below the readings 

 of the angles of incidence and the corresponding angles of diffraction 

 obtained with the different gratings, and the wave-length deduced 

 from them. 



Grating A. Breadth of strip = 1 cm. 



