1875.] the Observation of the Transit of Venus. 81 



for exposing the successive plates instead of having (as in the present slide) 

 to be reversed through an entire revolution, which alone takes nearly half a 

 minute. 



The photoheliograph, like all work turned out by Mr. Dallmeyer, 

 was an excellent and perfectly finished instrument, but seemed to me 

 to be scarcely sufficiently firmly mounted for continuous work ex- 

 tending over so many hours, with the constant shaking caused by the 

 insertion and withdrawal of the dark slides, which were much stiffer than 

 they ought to have been. This stiffness of the dark slides was found 

 not to be due to climatic influence, because they did not agree in 

 measurement with the focussing screen which fitted perfectly, and 

 they had to be filed down considerably before they would fit ; this 

 defect, due no doubt to an oversight in the maker or to hurry in turning out 

 the instrument, was a serious one, as besides the liability to tremor caused by 

 the frequent alteration of declination, the foeus might have been disarranged 

 by the alteration in the thickness of the slides by filing, but there was 

 nothing else to be done under the circumstances. 



For my part, speaking merely as a photographer, I should prefer the 

 system adopted by Lord Lindsay and the American parties in which the 

 camera was an immovable fixture and the solar image retained in a constant 

 position by means of a siderostat carefully adjusted to follow the sun. In 

 any case, the slides should be constructed to fit quite easily into their places, 

 and in this respect the dark slides made for the equatorial camera used at 

 Dodabetta for photographing the solar eclipse in 1871, were of a much 

 better pattern than those sent out with the photoheliograph. 



Another defect of the photoheliograph was that the hanging counter- 

 poise, placed near the object-glass of the telescope when using the Janssen 

 slide, was found to swing and induce a tremor in the instrument, spoiling the 

 definition of the pictures ; it was therefore replaced with a rough, but efficient 

 substitute, in the shape of a canvas bag, the ends of which were filled with 

 shot. This was merely hung over the end of the telescope at the proper 

 balancing point and kept the tube perfectly steady. 



As regards the process to be adopted for photographing the Transit of 

 1882 much will depend on the results obtained by the different methods 

 used in December last as to whether photography can be advantageously 

 employed and, if so, which process is most suitable. 



As far as my experience goes, the wet process seems less favourable to 

 perfect sharpness and clearness of the image than the dry, but Colonel Ten- 

 nant tells me he has lately obtained very superior results by using a pyro- 

 gallic acid developer with bromoiodised collodion, in place of the iron 

 development. From experience I have gained in preparing for photograph- 

 ing the recent Eclipse, I believe that great advantages may be obtained by 

 slightly staining the ordinary wet films with orange or red anilin dyes or 



