70 J. Waterhouse — "Photography in connection with [No. 2, 



the collodion film : more recently, however, Paschen had found this 

 contraction to amount to not less than y-g-yg- of the length of the plate 

 with alhumenised plates, and to j-tVt of unalbumenised plates ; in one 

 instance it being so much as T |-3 of the length and -g-^g- of the breadth of 

 the albumenised plate. Rutherfurd, on the other hand, found that if 

 the plates received a preliminary coating of albumen, the shrinkage of the 

 wet film in drying did not exceed -j-^/' and was, on an average, about five 

 times less. Prof. H. Vogel, of Berlin, also made some experiments on the 

 conditions affecting the stability of the collodion film, which proved the 

 value of a substratum as a preventive of contraction of the film and shewed 

 that dry plates were less liable to contraction than wet. Captain Abney 

 and Colonel Stuart Wortley, when experimenting on a dry process to be used 

 for the transit by the English expeditions, also gave this subject their care- 

 ful consideration and came to the conclusion that with proper precautions the 

 amount of shrinkage would be so small as to be negligible. Notwithstanding 

 this concurrence of testimony as to the possibility of disregarding the contrac- 

 tion of the film, I thought it desirable to satisfy myself as to the suitability 

 in this respect of the various dry processes I was trying, and the plates were 

 therefore tested by a method which I afterwards found was somewhat similar 

 to that followed by Dr. De la Rue, and appeared to have the advantage of 

 entirely avoiding any chance of error from parallax caused by want of absolute 

 contact between the test lines and the collodion film. Several glass plates 

 five inches square were prepared by drawing on them, with a very fine diamond 

 point, diagonal lines through the corners of the plates. With the intersec- 

 tion of the diagonals as a centre, a circle was described 4 in. in diameter, so 

 that it might correspond in size with the solar disc on the plates to be taken 

 during the Transit. These test plates were then coated with the usual 

 albumen substratum and prepared exactly in the same way as the dry 

 plates under trial. They were exposed to light from the- back, so that an 

 impression of the engraved lines was obtained through the film. The 

 plates were then developed in the same way as the other plates and when 

 dry, examined under a very powerful micrometer capable of dividing to the 

 _i-__ of an inch. To facilitate the examination, a piece of the film was 

 cut away across the lines in different parts of the plate, and the course of 

 the uncovered part of the line compared with the covered part. In no case 

 was any perceptible difference found, except when the substratum had been 

 purposely omitted, or processes used which gave rise to blistering of the 

 film. The only chance of error I could see in this plan was the sticking 

 of the film to the rough surface of the engraved lines ; but in the cases 

 where the film blistered it was found that the blistering was more marked 

 on the lines than elsewhere, and so it would appear that the lines did not 

 exert any particular influence on the free motion of the film. I had not 



