134 report — 1859. 



even moderate success if he dabbles in celestial photography in a desultory 

 manner, as with an amusement to be taken up and laid aside. 



In order to prosecute celestial photography successfully, there must be, 

 in close contiguity with the telescope, a Photographic Room, abundantly 

 supplied with both common and rain water. The water-taps should pro- 

 ject over a sink, so as to reach about a foot from the wall. The rain 

 water is conveniently kept in and filtered by an ordinary stone-ware filter. 

 The photographic room may be lighted generally by means of an ordinary 

 Argand reading lamp, over the shade of which hangs a lantern-like curtain 

 made of two thicknesses of deep-yellow calico ; but the plate, during the 

 development of the picture, must be illuminated locally by a night-light before 

 which a yellow screen is placed. The photographic room should be furnished 

 with a stove, burning wood or charcoal, which will keep alight for a long 

 time, in order that its temperature may never fall much below 50° F. during 

 the winter. 



In my earlier experiments, the positive process was invariably employed 

 on account of its greater rapidity ; but so many details, visible by trans- 

 mitted light in a positive, are lost when it is afterwards viewed by reflected 

 light, that endeavours were made to render the negative process equally 

 rapid. After many trials, I succeeded in this ; and I now never have 

 recourse to the positive process, except for some special object. 



Glass used. — It is of course necessary to have the plate somewhat larger 

 than the object to be taken ; the size used when the telescope is employed as 

 a Newtonian is 2f inches by 3^- inches. When the pictures are taken by 

 the direct method, the plates are circular, and 2f- inches in diameter. 

 The outside diameter of the slide to contain the circular plate is 3^ inches, 

 the exact size of the cell of the diagonal mirror, so that no more light is 

 stopped out by the plate-holder than by the small mirror. 



The glass used is the " extra white patent plate," and I have it selected as 

 free from specks and bubbles as possible, but nevertheless I have frequently 

 to reject about one-third of those discs which are supplied to me. 



Mode of Cleaning the Plate. — The glass is cleaned in the ordinary way by 

 means of tripoli powder, mixed up with three parts of spirit of wine and one 

 of liquid ammonia, to the consistence of cream. For drying the plates I am 

 provided with two* cloths, which, in the first instance, have been carefully 

 washed with soda (avoiding the use of soap), and repeatedly rinsed in water. 

 Each time after being used, these cloths are thoroughly dried, but they need 

 not be washed for months together. For the final wiping of the plate a piece 

 of wash-leather is employed, also carefully dried before being used. 



A piece of grit-stone, such as is used by mowers to sharpen scythes, must 

 be at hand, for the purpose of grinding the edges of the glass plate and 

 making scratches on the margin of the two surfaces, in order to cause the 

 more perfect adherence of the collodion, 



The plate to be cleaned is placed on a sheet of cartridge paper, and rubbed 

 thoroughly, first on one side, then on the other, with a piece of new cotton- 

 wool moistened with the tripoli mixture, above described. It is then washed 

 in a stream of water, the fingers being used, if necessary, to aid in removing 

 the adhering tripoli. Holding the plate while still wet, and without touching 

 the surface, one edge after the other is rubbed on the grit-stone ; the glass 

 imbeds itself in the friable stone, and thus the borders of the two surfaces 

 get scratched, and the edge is ground at the same time. After the four 

 edges have been so ground, or, if the plate be circular, the whole periphery ■ 

 has been rubbed, the hands and plate are well washed, to remove all grit, and 

 the plate placed edgewise for a few seconds on a marble slab. With dry 

 * It is disadvantageous to employ more cloths than are absolutely necessary. 



