and on some Phenomena connected therewith. 197 



glass surface, faced with highly reflecting silver. Since the 

 optical depth of the lines is increased some four times, these 

 gratings usually give much brighter spectra than the glass 

 originals. 



Prof . Quincke was kind enough to send me some specimens of 

 his work, giving extremely beautiful spectra. I found, how- 

 ever, that, though carefully preserved, these gratings deterio- 

 rated after a time, apparently either from insufficient thickness, 

 or from imperfect adhesion of the silver layer. In my own 

 attempts I endeavoured to remedy this defect by not allowing 

 the silver to dry before transference to the electrolytic cell, 

 and by commencing the electric deposit with a silver instead 

 of with a copper solution. I did not, however, succeed in 

 finding a thoroughly satisfactory plating-liquid. In the 

 ordinary cyanide solution the silver was at once loosened from 

 the glass. In other solutions the grating could be immersed 

 with impunity, but the film began to strip as soon as the 

 current passed. Using acetate of silver, however, I was able 

 to obtain a certain degree of thickening. I also found ad- 

 vantage from commencing the deposit of copper with a 

 neutral solution. After the layer had attained a moderate 

 thickness, its edges were dipped in melted paraffine ; and it 

 was then transferred to the usual acid solution of copper. 

 I did not find it necessary to take any precautions against 

 too great an adhesion between the silver and the glass. 



These copies are now four years old, and they do not seem 

 to have deteriorated. A slight yellow tarnish, due probably 

 to sulphur, can be removed with cyanide of potassium. There 

 is, however, one defect which I have not been able to avoid. 

 The silver surface is never sufficiently flat to bear much mag- 

 nifying-power. Unless this difficulty can be overcome, the 

 use of such gratings must be limited to cases where brilliancy 

 and not high defining-power is the desirable quality. For 

 most purposes the photographic method of reproduction is to 

 be preferred as far easier and quicker. Among various pro- 

 cesses of this kind, I am still inclined to give the preference 

 to that in which collodio-chloride of silver is employed, with 

 subsequent treatment with mercury. The only trouble that 

 I have met with is the tendency of the soluble salts to crys- 

 tallize in the film ; but this can generally be avoided with a 

 little judgment. As these photographs cannot well be 

 varnished, some doubts might have been entertained as to their 

 permanence ; but I find that copies now more than seven 

 years old are none the worse. For gratings to be subjected 

 to rough treatment, the various albumen processes offer 

 decided advantages. 



