252 Capt. W. de W. Abney and Mr. G. S. Edwards. 



That the former are not in great error will be seen by comparing 

 the place of maximum density of the former with the place of maxi- 

 mum sensitiveness of the latter. 



It may be well to remark here on one point to which objection 

 might be taken in the results. It has been assumed in making the 

 scale that length of exposure is equivalent to the intensity of the 

 light. This is not a hasty assumption, but has been carefully tested. 

 When the exposure is but the minute fraction of a second, then the 

 substitution of length of exposure for intensity does not apparently 

 hold good ; but, when the exposures are such as are given to the 

 scale, the substitution is perfectly legitimate. 



In the following tables the curves of the simple haloid salts of 

 silver are given, as well as mixtures of two or more, and also double 

 salts. Where double salts of silver are shown they were prepared 

 by mixing the alkaline salts in proper equivalent proportions, and 

 then emulsifying in gelatine or collodion by adding the requisite 

 amount of silver nitrate to them. Where simple mixtures are shown, 

 emulsions containing the proper equivalent amount of the silver salts 

 were prepared and subsequently mixed. It has been deemed desirable 

 to give the values for the haloid salts when stained with certain dyes, 

 such as are usually employed in rendering photographic plates what is 

 termed isochromatic. Attention is called to the fact that a mixture 

 of solutions of two dyes does not render the salts of the same sensi- 

 tiveness to different parts of the spectrum as do the two dyes if 

 applied separately, washing taking place between the application of 

 each. For erythrosin Mailman's well known formula was used to 

 obtain the coloured solution. The erythrosin was obtained from 

 Germany, and showed only traces of fluorescence. The cyanin was 

 obtained from Messrs. Hopkin and Williams, and appears to be made 

 after Greville Williams' formula. When cyanin was employed, 5 grains 

 was dissolved in 1 oz. of alcohol, and water added to make up to 

 2 ozs. This solution was poured over the plate, which was then 

 allowed to dry. The plate was then washed with equal parts of spirit 

 and water, and "finally with water, and then exposed to the spectrum. 

 Similar results were obtained whether the film was dry or moist. 



[In all the tables except VII, VIII, IX, X, XV, and XVI, the 

 following are the points on the scale numbers of the principal 

 Fraunhofer lines: H, 13-8; G, 10*9; Li, 9; F, 7'6; E, 6*0; D, 4*3; 

 Li, 2-8. 



In Tables VIII and IX the following are to be substituted : 

 H, 14-0; G, 110; Li, 9-0: F, 77; E, 6*2; D, 4*6; Li, 31. 



In Tables VII, X, XV, and XVI the following are to be substi- 

 tuted : H, 67; G, 9; Li, 107; F, 121 ; E, 13; D, 14-3; Li, 157. 

 —Feb. 10, 1890.] 



