[176] PROFESSOR STOKES, ON THE COLOURS OF THICK PLATES. 



intensities, but are decidedly more brilliant on the side of the image than on the side of the 

 flame itself. That this is not due merely to the glare of the direct light, may be proved 

 by holding a small object in front of the flame, so as to screen the eye from the direct light, 

 when the rings, though better seen than before in the neighbourhood of the flame, are still much 

 weaker than on the opposite side, if the distance of the flame from the axis is at all consider- 

 able. For the same reason, in the case of a plane mirror, when the luminous point is placed 

 a good distance in front of the eye, so that the rings do not run out of the field of view, 

 they cannot be traced throughout their whole extent if the angular distance between the 

 luminous point and its image be too great, but only throughout a portion, more or less 

 considerable, on the side of the image. 



38. In the case of a concave mirror when the luminous point is not far from the centre 

 of curvature, and the rings are viewed by an eye placed at no great distance off", the first 

 factor in the expression for fr (equation 38) is not large, and the angle of diffraction does not 

 increase rapidly in passing away from the image. In the case of a plane mirror o«eo, and 

 if we suppose c and h equal to what they were in the former case, or thereabouts, in order to 

 make the two cases comparable in every respect except the curvature of the mirror, the factor 

 in question, though larger than before, is still sufficiently small to prevent $ from increasing 

 very rapidly on receding from the image. Accordingly, in both these cases, the rings and 

 bands are seen with brilliancy at a considerable angular distance from the image. But in the 

 case of a convex mirror of considerable curvature p is negative, and not large, so that the factor 

 in the expression for 5 2 becomes considerable, and accordingly the angle of diffraction increases 

 rapidly on receding from the image. I found, in fact, that such a mirror was peculiarly 

 ill suited for producing rings or bands, inasmuch as only a comparatively small portion 

 of the system usually seen was visible, namely, the portion which lay in the immediate 

 neighbourhood of the image. 



G. G. STOKES. 



