Celestial Pliotograpfaj. 137 



-po grain, water 1 drachm "(this part of the process is never 

 employed by Mr. De la Ene) . Protochloride of palladium, he 

 found, would increase the intensity of a negative sixteen times 

 without any injury to the image, or the production of markings, 

 and is only kept out of general use, he says, by the scarcity of 

 the metal. 



The shortest exposure was one-third of a second, the moon 

 being twenty-one days old, but the sky singularly clear. The 

 full moon would, under such circumstances, have required a 

 much shorter exposure, the focal image being exceedingly 

 brilliant. Occasionally, however, a reverse condition of the 

 sky obtains for a month or six weeks, at least in America, 

 when the pale-yellow moon emits non-actinic rays. This, 

 which occurs in very dry weather, Dr. Draper is disposed to 

 refer to dust in a state of minute division. In such a condition 

 of air, the full moon depicted itself with only moderate 

 intensity in twenty seconds; that is to say, forty times as long 

 as usual. 



The enlargement of the picture is effected by means of a 

 concave mirror of 9 inches aperture, and 11^ inches focus (for 

 parallel rays), worked to an elliptical figure of 8 feet distance 

 between the conjugate foci, and intended to magnify seven 

 times. At first the negative was illuminated by diffused day- 

 light, and the entire aperture of the mirror used; but the 

 whole image was not reproduced equally sharp in every part. 

 Subsequently, by means of a heliostat, a solar beam 1^- inch 

 in diameter was passed through the negative to the mirror, a 

 little on one side of its vertex, and reflected back on the 

 sensitive collodion plate at a distance ; and it was found that, 

 when so small a portion of the concavity was employed, an exact 

 correspondence of the conjugate foci with theory was not 

 demanded, and the mirror performed equally well, whether 

 magnifying seven or twenty-five times. He has even suc- 

 ceeded with a photograph of the moon 50 inches in diameter, 

 which has, as might be expected, a very imposing effect. 



Dr. Draper adds some remarks on microscopic photography, 

 in which the employment of a cell of ammonio-sulphate of 

 copper proved very successful. This deep blue liquid was 

 found by his father to have the property of transmitting only 

 the more refrangible rays, with the advantage of a much larger 

 beam of illuminating light without burning delicate objects, 

 and also of much higher powers ; and thus in 1856 they ob- 

 tained excellently defined photographs of frog's blood- discs, 

 Navicula angulata, and several similar objects, under a power of 

 700 diameters. 



When our readers find that on some nights as many as 

 seventeen lunar negatives were taken, most of which were 



