Theory of Diffraction-gratings. 87 



The exposure may be from two to six minutes to the direct rays 

 of the sun.- I have not been able to detect any deterioration 

 when the plates were kept a few days in the dark before being 

 used. 



tV photographer accustomed to either the plain albumen or the 

 Tanpenot process will find it very suitable for gratings. The hard- 

 ness of the surface, which allows varnish to be dispensed with, 

 is a great advantage. In my experiments with plain albumen, 

 the principal difficulty was the purely photographic one of avoid- 

 ing stains. It must be observed, however, that in actual use the 

 gratings are not seen in focus, and that excellent spectra may be 

 obtained from copies which a photographer would be inclined to 

 throw away at once as hopelessly faint and dirty. The objection 

 to the Taupenot process is the trouble of preparing the plates ; 

 but this is much mitigated when the plan is adopted of prepa- 

 ring large pieces to be afterwards cut up. 



Among those requiring development, the tannin process is the 

 one with which I have been most successful. In order to coun- 

 teract the well-known tendency of the film to loosen, a prelimi- 

 nary coating of dilute albumen or gelatine is generally necessary. 

 In the production of gratings the photographer must not be 

 satisfied with merely keeping the film on the glass ; the slight- 

 est tendency to looseness must be considered highly objection- 

 able. The plates are coated with Mawson J s collodion, excited 

 in an ordinary silver-bath, washed first in distilled water and 

 afterwards under the tap, and then immersed for a minute in 

 a well-filtered 15-grain solution of tannin. On removal from 

 the tannin, they are set up cornerwise on blotting-paper to drain 

 and dry. 



For the development of these plates I prefer a solution of gallic 

 acid employed in a dish, though I have obtained very good 

 results by the ordinary method with pyrogallic acid. Prepare 

 the two following : — 



(1) Gallic acid .... 100 grains. 

 Alcohol ..... 2^ oz. 

 Distilled water ... 2i oz. 



(2) Nitrate of silver . . 100 grains. 

 Glacial acetic acid . . 2 oz. 

 Distilled water . . . 16 oz. 



The dish used for developing should be of glass, and is best 

 cleaned with a little strong nitric acid, which may be used over 

 and over again. The developing solution is prepared by mixing 

 (1) and (2) in equal parts and diluting with water to half the 

 strength. The alcohol helps to keep the film tight; and the de- 

 velopment is well under control. In warm weather the opera- 



