138 REPORT — 1859. 



in a small vessel, and to place it in readiness, before the plate is removed from 

 the bath and put into the slide, so as to prevent any delay after the plate 

 has been exposed in the telescope. This precaution obviates the staining 

 which arises sometimes by partial drying of the film. 



The addition of nitrate of silver to aid in bringing out the picture must 

 be avoided ; pictures thus intensified will not bear any magnifying power, 

 and are comparatively worthless. Hence it will be seen how all-important 

 it is to have the bath and collodion in their most sensitive condition. The 

 negative should not be developed too strongly, as such pictures never copy 

 so well as those moderately but distinctly brought out. Such small photo- 

 graphic pictures as those of Jupiter and Saturn present many obstacles to 

 their development, on account of the difficulty of discerning them during 

 the operation ; for the focal image of Jupiter in my telescope, even when 

 the planet is in opposition, is only about -^th of an inch in diameter. 



After the development of the picture to the desired point, the further 

 development is arrested by pouring a quantity of water on the plate, and a 

 vessel containing water should be at hand for this purpose. 



Fixing the Picture. — By preference I use hyposulphite of soda for fixing ; 

 after fixing, the plate is washed under the tap of a cistern of water for a short 

 time, and then examined with a lens. If worth retaining, the epoch of the 

 picture, and other particulars are recorded at the back with a writing dia- 

 mond. The plate is then washed again, front and back, in a stream of water, 

 and placed face upwards on a tripod stand, duly levelled ; rain-water* is 

 poured on the collodion, and from time to time this is poured off and fresh 

 poured on, in the meantime other photographs are proceeded with. After 

 half an hour or more, the plate is thoroughly washed in a stream of rain- 

 water, and placed edgewise on blotting-paper against the wall, to drain and 

 dry. 



Varnishing. — The next morning, the negatives are warmed before a fire, 

 and varnished with Scehnee's varnish f, which is the only description I have 

 found to stand. I am careful to filter the varnish before using; otherwise 

 specks might be transferred to the photograph. It is very desirable to var- 

 nish the plates as soon as they are dry, for, if left unvarnished for any 

 length of time, they can never be varnished evenly. 



Desiderata in the Machinery for driving the Telescope. 



As in the production of celestial photographs some seconds of exposure 

 are requisite, it is essential to have a clock-work driver to the telescope, 

 which works uniformly and smoothly, and which is also capable, when lunar 

 pictures are to be taken, of ready adjustment to the ever-varying lunar time. 

 Lunar time, it will be recollected, differs from sidereal time, in consequence 

 of the moon's variable motion in her orbit in a direction opposite to that of 

 the apparent diurnal movement of the stars. A driving clock, if adjusted 

 to follow a star, must be retarded therefore, more or less, in order to follow 

 the moon. In my own telescope, this is at present effected by altering the 

 length of the conical pendulum or friction governor, thus altering the time 

 of its rotation (or double beat), and this plan, or some modification of it, is 

 universal. My experience, however, has pointed out several inconveniences 

 in thus changing the speed of the governor or pendulum, and it is my intention 

 to make such alterations in the construction of the clock as will enable me 



* In preparing the bath and developing solutions, distilled water must be employed, 

 but filtered rain-water answers very well for washing the photographs. 

 f Sold by Messrs. Gaudin, 26 Skinner Street, London. 



