38 PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 



miss one or more observations, corresponding to certain readings of the polarizer, or else to 

 alter a little the direction of the incident light, or, by means of the screws, to turn the grating 

 through a small angle round a horizontal axis. The deviation of the light which passed 

 through the polarizer, and the small changes in the direction of the incident light, I regard as 

 the chief causes of error in my experiments. In repeating the experiments so as to get 

 accurate results, these causes of error would have to be avoided. 



At first I took for granted that the instrument-maker had inserted the doubly refracting 

 prism in the polarizer in such a manner that the plane of polarization of the less refracted 

 pencil was either vertical or horizontal, (the instrument being supposed to stand on a horizontal 

 table,) when the pointer stood at 0°, having reason to know that it was not inserted at random ; 

 and having determined which, by an exceedingly rough trial, I concluded it was vertical. 

 Meeting afterwards with some results which were irreconcilable with this supposition, I was 

 led to make an actual measurement, and found that the plane of polarization was vertical when 

 the pointer stood at 25°. Consequently 25° is to be regarded as the index error of the polarizer, 

 to be subtracted from the reading of the pointer. The circumstance just mentioned accounts 

 for the apparently odd selection of values of tjt in the earlier experiments, the results of which 

 are given in the tables at the end of this section. 



On viewing a luminous point or line through the grating, the central colourless image was 

 seen accompanied by side spectra, namely, the spectra which Fraunhofer called Spectra of the 

 second class. After a little, these spectra overlapped in such a manner that the individual 

 spectra could no longer be distinguished, and nothing was to be seen but two tails of light, 

 which extended, one on each side, nearly 90° from the central image. On viewing the flame of 

 a spirit lamp through the grating, the individual spectra of the second class could be seen, 

 where, with sun-light, nothing could be perceived but a tail of light. The tails themselves 

 were not white, but exhibited very broad impure spectra ; about two such could be made out 

 on each side. These spectra are what were called spectra of the first class by Fraunhofer, 

 who shewed that their breadth depended on the smaller of the two quantities, the breadth of a 

 groove, and the breadth of the polished interval between two consecutive grooves. In the 

 grating, the breadth of the grooves was much smaller than the breadth of the intervals between*. 



In the experiments, the diffracted light observed belonged to a bright, though not always 

 the brightest, part of a spectrum of the first class. The compound nature of the light was 

 easily put in evidence by placing a screen with a vertical slit between the grating and the eye, 

 and then viewing the slit through a prism with its edge vertical j. A spectrum was then seen 

 which consisted of bright bands separated by dark intervals, strongly resembling the appear- 



• On viewing the grating under a microscope, the grooves 

 were easily seen to be much narrower than the intervals be- 

 tween ; their breadth was too small to be measured. On look- 

 ing at the flame of a spirit lamp through the grating, I counted 

 sixteen images on one side, then several images were too faint 

 to be seen, and further still the images again appeared, though 

 they were fainter than before. I estimated the direction of zero 

 illumination to be situated about the eighteenth image. If we 

 take this estimation as correct, it follows from the theory of 

 these gratings that the breadth of a groove was the eighteenth 



part of the interval between any point of one groove and the 

 corresponding point of its consecutive, an interval which in the 

 case of the present grating was equal to the l-1300th part of an 

 inch. Hence the breadth of a groove was equal to the l-23400th 

 part of an inch. 



f To separate the different spectra, Fraunhofer used a small 

 prism with an angle of about 20°, fixed with its edge horizontal 

 in front of the eye-piece of the telescope through which, in his 

 experiments, the spectra were viewed. 



