A SILVERED GLASS TELESCOPE. 47 



nmnins into a tank /, Fig. 38, which will contain a ton weight. The roof exposes 

 a .surface of 582 sciuare feet, and consequently a fall of rain equal to one inch in 

 depth, completely liUs the tank. During the course of tlu- year the fall at tliis place 

 is about :32 inc-hes, so that tlieri^ is always an abuufUmce. In order to keep the 

 water free from contamination, tlie roof is painted with a ground mineral coin])()iind, 

 which hardens to a stony cf)nsistence, and resists atmospheric influences well. 'I'lic 

 tank is IuuhI with lead, but luiving lieen in use for many years for other piu-poses, is 

 thoroughly coated inside with various salts of lead, sulpliates, &c. In addition the 

 precaution is taken of emptying the tank by a large stopcock when a rainstorm is 

 approaching, so that any accumulation of organic matter, which can reduce nitrate 

 of siher, may be avoided. It has not been found feasible to use the well or spring 

 water of the vicinity. 



The tank is placed close under the eaves of the bnikUng, so as to gain as much 

 liead of water as is desirable. From near its bottom a pipe terminating in a stop- 

 cock /.■, Fig. 38, passes into the Laboratory. In the nortlieast corner of the room, 

 and under the tap is a sink for refuse water and solutions, and over which the 

 negati\( s are developed. It is on an average about twelve feet distant from the 

 teles(H)pe. In another corner of the room is a stove, resembling in construction an 

 open fireplace, but sufficient nevertheless to raise the temperature to 80° F. or higher, 

 if necessary. As a pro\ision against heat in sunnner, the walls and roof are double, 

 and a free space with numerous openings above is left for circulation of air, draAvn 

 from the foundations. Tlie roof is of tinplate, fastened directly to the rafters, with- 

 out sheathing, in order that heat may not accumulate to such an extent during the 

 day as to constitute a source of disturliance when looking across it at night. 



For containing negatives, wliicli from being unvarnished require particular care, 

 there is at one side of the room a case with twenty sliallow drawers each to hold 

 eighteen. They accumulate very rapidly, and were it not for frequent reselections 

 the case would soon be filled. On some nights as many as seventeen negatives have 

 been taken, most of which were worthy of preservation. Not less than 1500 were 

 made in 1862 and '63. 



b. Pliotogi-aphic Processes. 



In photographic manipulations I have had the advantage of my father's long 

 continu(>d experience. He worked for many years with bromide and chloride of 

 silver in his photo-chemical researches (Journal of the Franklin Institute, 1837), 

 and when Daguerre's beautiful process was published, was the first to ajjply it to 

 the taking of portraits (Phil. Mag., June, 1840) in 1830; the most inqxirtant of all 

 the applications of the art. iSubse([uently he made ])hotogra])hs of the intertercMue 

 spectrum, and ascertained the existence of great grou])s of lines J/, ^V, O, P, above 

 H, and totally invisible to the naked eye (Phil. Mag., May, 1843). The importance 

 of these results, and of the study of the structurr' of iiames containing various 

 elementary bodies, that he made at the sam(> time, are only noAV exciting the 

 interest they deserve. 



In 1850, wluni his work on Physiology was in [)reparation, and the numerous 

 illustrations had to be i)ro(luced, I learnt microscopic photography, and soon after 



