136 Celestial Photography. 



or less beneath, the surface of oil or mercury. But these 

 contrivances involve an expensive train of wheel- work, and it 

 seems likely that one of Dr. Draper's prime movers would be 

 found to answer with a much more simple arrangement ; and 

 it is on this account that they have been here described with 

 so much detail, since they appear well worthy of the attention 

 of the possessors of equatorial instruments. 



In solar photography, the very short exposure required is 

 secured, as on this side of the Atlantic, by a movable shutter, 

 drawn by an india-rubber spring ; this acts with such rapidity, 

 that " the wavy appearance produced by atmospheric disturb- 

 ance is not unfrequently observed sharply defined in the 

 photograph, though, these aerial motions are so rapid that they 

 can scarcely be counted." To avoid this very short exposure, 

 many experiments were made with an unsilvered mirror, 

 reflecting, according to Bouguer, only 2| per cent, of incident 

 light ; but the plan had to be abandoned, from the change of 

 figure and focal length immediately resulting from exposure to 

 the sun. It was found that a glass mirror, though only resting 

 on a few points, can be raised to 120° Fahr. by putting it in 

 the sunshine on a hot day. An unsilvered " flat," or plane 

 mirror, was found to answer better. But the ingenious in- 

 ventor considers that the " best means of acquiring fine results 

 with solar photography would be to use the telescope as a 

 Cassegrainian, and produce an image so much enlarged, that 

 the exposure would not have to be conducted with such 

 rapidity." 



Dr. Draper found that the pinholes, coarseness of granula- 

 tion in the reduced silver, liability to stains and markings, and 

 spots produced by dust, which assume an offensive importance 

 in enlarging the portrait, were all avoided by washing off the 

 free nitrate of silver from the sensitive plate before exposing it 

 to the light, and again submitting it to the action of water, 

 and dipping it back into the nitrate bath before developing : 

 the quantity of nitrate of silver, however, necessary to de- 

 velopment when pyrogallic acid is used, he says, is better 

 procured by mixing it, as usual, with the acid. His- nitrate 

 bath contains 40 grains to the ounce, and is acidified till it 

 reddens litmus paper. Focussing takes place on the sensitive 

 collodion itself, the actinic rays being cut off by a yellow glass 

 just in front of it. A pasteboard screen is then placed in front 

 of the telescope, and the yellow glass taken out ; and after a 

 lapse of twenty seconds (to obviate, presumably, any accidental 

 tremors), the screen is removed for the operation. He de- 

 velopes with protosulphato of iron 20 grains, acetic acid 

 1 drachm to the ounce ; and intensifies, if necessary, with 

 pyrogallic and citric acid each ^- grain, nitrate of silver 



