AS I LVERED GLASS TELESCOPE. 47 



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

 a surface of 532 square feet, and consequently a fall of rain equal to one inch in 

 depth, completely fills the tank. During the course of the year the fall at this place 

 is about 32 inches, so that there is always an abundance. In order to keep the 

 water free from contamination, the roof is painted with a ground mineral compound, 

 which hardens to a stony consistence, and resists atmospheric influences well. The 

 tank is lined with lead, but having been in use for many years for other purposes, is 

 thoroughly coated inside with various salts of lead, sulphates, &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 silver, 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 building, so as to gain as much 

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

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

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

 negatives arc developed. It is on an average about twelve feet distant from the 

 telescope. 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 provision against heat in summer, the walls and roof are double, 

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

 from the foundations. The 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 disturbance when looking across it at night. 



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

 there is at one side of the room a case with twenty shallow 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. Photographic Processes. 



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

 continued 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 apply it to 

 the taking of portraits (Phil. Mag., June, 1840) in 1839 ; the most important of all 

 the applications of the art. Subsequently he made photographs of the interference 

 spectrum, and ascertained the existence of great groups of lines M, N, 0, P, above 

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

 of these results, and of the study of the structure of flames containing various 

 elementary bodies, that he made at the same time, are only now exciting the 

 interest they deserve. 



In 1850, when his work on Physiology was in preparation, and the numerous 

 illustrations had to be produced, I learnt microscopic photography, and soon after 



