1875.] the Observation of the Transit of 'Venus. 69 



uniformity will be attained by the use of materials which are likely to be 

 of nearly the same chemical composition in all parts of the world. 



As the beer-albumen process was not found to answer, attention was 

 turned to other dry processes and several different methods were tried with 

 varying results. 



At an early stage of the experiments it was found from trials with a 

 rough photoheliograph, constructed in Calcutta for the purpose, that a pro- 

 cess which might give very good results for taking views &c. would not 

 answer for the sun and vice versd ; and the same was afterwards found to be 

 the case when working with the English photoheliograph. 



Among the most promising dry processes tried in these preliminary 

 experiments were the gum-gallic, in which the so-called preservative is 

 composed of a solution of gum arabic and gallic acid, and a process in 

 which the preservative was laudanum, either alone, as a dilute solution 

 in water containing from 16 to 4 per cent, of laudanum, or mixed with 

 gum arabic or gum tragacanth, in order to keep the pictures free from the 

 stains liable to occur when using the laudanum alone. Excellent results 

 for views were also obtained with a filtered mixture of laudanum and very 

 thin arrowroot water. I was induced to use the laudanum from a state- 

 ment of Prof. Vogel of Berlin, that plates prepared with morphia were 

 more sensitive to the comparatively non actinic rays from the outer part of 

 the solar disc ; and though I did not remark any special superiority in 

 this respect, the laudanum plates were found more sensitive than most of 

 the others tried. Plates prepared with a saturated solution of morphia in 

 water also gave good results. 



The addition of nitrate of uranium to the nitrate of silver bath used 

 for sensitising the plates, as recommended by Captain Abney, was found 

 advantageous for most of the dry plates, giving increased sensitiveness and 

 other good qualities. As some doubt has lately been thrown on the advan- 

 tage of the uranium bath, it may be as well to state that in the ordinary 

 wet process with bromo-iodised collodion I have found that no advantage is 

 gained by the addition of the uranium salt to the nitrate bath, but, on the 

 contrary, there is a great loss of sensitiveness. With dry plates, however, 

 it is different, the gain in sensitiveness is well-marked and the shadows 

 appear cleaner than on plates sensitised in the ordinary bath without the 

 uranium. 



Shrinkage of the Collodion films. — When it was first proposed to 

 employ photography in observing the Transit, it was objected that the 

 collodion processes would be unsuitable on account of the shrinkage or 

 contraction the collodion films undergo in drying. De la Rue in 1861 

 made some very careful experiments, the result of which was to shew that 

 with proper precautions the shrinkage was entirely in the thickness of 



