The President's Address. By H. G. Sorhj, F.R.S., &c. Ill 



rays at the blue end of the spectrum are those which are active in 

 photographing, it might be possible to obtain a good photograph of 

 lines not distinctly visible when mixed light is employed. This, 

 Helmholtz thinks, explains why Stinde was able to photograph lines 

 on SurirelJa gemma which were rooVoo- of an inch apart, and 

 therefore considerably within the possible limit. Helmholtz does not 

 appear to have seen the papers on Nobert's bands by Stodder * and 

 by Dr. Woodward,! which contain many facts of great interest in 

 connection with this subject. 



In reading these papers it is easy to perceive that the true reso- 

 lution of one of Nobert's bands, which according to Dr. Woodward 

 contains lines at a distance of about rr^oo-o of an English inch, is 

 a matter of such extreme difficulty, even with the best object- 

 glasses, that, if the exact nature of the object and the number of 

 lines were not known, it would be almost impossible to decide how 

 many lines there were to an inch. The lines due to interference 

 are often as distinct as the true lines on the glass, and Dr. Wood- 

 ward believes that such spurious lines, and not the actual, were 

 seen and counted by Stodder ; since the number was not correct. 

 The black lines due to interference do occur beyond the limit of 

 the true, and at closely the same intervals as the real, as should be 

 the case according to Helmholtz's theory. Now it is quite manifest 

 that the distinctness of definition depends on how these spurious 

 bands occur in relation to the true. If they exactly overlap, the 

 definition would be good, and the lines distinct ; but, if they 

 occurred at the half intervals, the dark part of one series occurring 

 at the bright part of the other would more or less completely 

 obliterate both. It appears to me very probable that these facts 

 will in great measure explain the phenomena seen when the light 

 is thrown on the lines at a varying angle, since in one position the 

 lines cannot be defined, on increasing the obliquity false lines are 

 visible, and with still more oblique light the true may be seen. An 

 alteration in the angle of aperture of the condenser would also alter 

 the distance of the dift'raction bands ; and therefore, taking all these 

 facts into consideration, we may easily explain why, as Helmholtz 

 says, it is possible under such favourable conditions, with lines at 

 equal intervals, to distinguish them when closer together than what 

 is the normal limit of the distance at which they can be seen 

 without any special difficulty, even when not at equal intervals, 

 that is to say, when the intervals are greater than that of the bands 

 due to diflraction. Even then, however, they do occur in varying 

 numbers and position between the true lines, as may be seen in 

 photographed diffraction gratings. 



* ' Quart. Jouru. of Micros. Science,' 18G8, vol. viii., p. 133. 

 t Ibid., p. 225; ' Moutlily Microscopical Journal,' 1871, vol. vi., p. 2G; aud 

 1872, vol. viii., p. 227. 



