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



its way on to the plate between the revolving disc and the wooden case, 

 which were at a greater distance apart than appeared necessary, though the 

 entrance of light might have been prevented by fitting the exposing disc 

 with a flange running in a groove cut in the wood-work of the slide or fastened 

 above it. There was also considerable friction about the internal sur- 

 faces, which caused a strain on the clock-work and gave a good deal 

 of trouble till the cause had been removed. With the exception of these 

 defects, the slide seemed admirably constructed and adapted for the 

 object in view. It remains, however, to be seen how far this ingenious 

 instrument has answered the expectations of its inventor and those who 

 have adopted it, but if it should be used at the next Transit, it would, I 

 think, be desirable that arrangements should be made for the automatic 

 movement to be continued or distributed at intervals over a much 

 longer period than one minute, as on the present occasion, so that 

 all the phenomena attending the contact may be fully observed and 

 recorded. It is also very desirable that the photographer should 

 not require a skilled observer to watch the time of contact for him. The 

 doing so has a very disturbing effect on a man who is able to make a good 

 observation of contact, and time is also lost in preparing and waiting for 

 a signal. 



As far as shewn by the plates obtained at Boorkee the differences be- 

 tween pictures taken a few seconds apart are so slight, and the advance of the 

 planet is so imperceptibly marked, if indeed, there is not sometimes an appear- 

 ance of retrogression caused by atmospheric tremor, that perhaps little would 

 be lost by taking the pictures at intervals of 4 or 5 seconds instead of at 

 every second. 



The mounting of the slide necessitates the alteration of the adjust- 

 ments of the telescope for taking the six-inch plates, thus stopping all such 

 observations about the critical period and it is therefore most desirable that 

 each operation with the Janssen slide should extend over as long a period 

 as possible. Colonel Tennant tells me that the cusp measures are indefinitely 

 more valuable, if good, than any six-inch plates, which he would entirely 

 eliminate. In this case, if it were considered essential that the successive 

 pictures should be taken at intervals of not more than one or two seconds, a 

 second, or even a third, Janssen slide might be provided so that they might 

 be rapidly changed one after the other. If it were feasible to construct the 

 slide so that the plates could easily be changed without removing the whole 

 slide from the camera, it would be better still, as in that case the observations 

 could be carried on at every second or two, and three or four plates exposed in 

 quick succession during five or six minutes about the time of contact, and, if 

 desirable, continued at regular intervals afterwards ; but this appears to pre- 

 sent considerable mechanical difficulties and an arrangement would be required 

 by which the revolving disc could be at once brought into the proper position 



