254 Photographic Use of a Silvered- Glass Reflecting Telescope. 



being developed with sulphate of iron, is brought out with pyro- 

 gallic acid, it assumes a much more brilliant aspect, the contrast 

 of light and dark parts is greater, and the picture more lively. 

 In enlarging such a photograph, by treating the reverse or posi- 

 tive made from the original negative with iodide of mercury 

 dissolved in iodide of potassium, the contrast can be increased 

 at pleasure, the dark parts of the collodion film gaining more 

 than their due proportion of strength. The eventual paper proof, 

 instead of being flat, is whiter in the high lights and darker in 

 the deep shades than it should be. Such a result shows the 

 uncertainty of any method of comparing the actinic force of 

 various parts of the moon together, and attempting to deduce 

 therefrom arguments as to their composition. The nature of 

 the collodion and sensitizing compounds used makes also a ma- 

 terial difference. I have found that, as a rule, it is better to use 

 sulphate of iron alone in those phases where the moon is about 

 one-half illuminated, and graduate the strength of the solution 

 so as to gain the most desirable result. The parts of the surface 

 on which the sunlight is falling nearly perpendicularly, are only 

 too prone to become overdark as compared with those on which 

 the light strikes obliquely. The contrast requires restraint 

 rather than encouragement. The negatives should present the 

 appearance of overdone positives to enlarge well. Even with 

 the best system of development, it is impossible to secure a good 

 picture of the entire portion of the moon visible in the first and 

 third quarters ; for either the larger portion must be overdeve- 

 loped and overexposed to secure the extreme edge, or else the 

 most obliquely illuminated part must be left undeveloped. It 

 may be possible to overcome this difficulty in the future, by 

 diaphragms so adjusted as to cut off the brighter light while the 

 fainter parts continue to act. 



In any operation of forcing or intensifying, it is necessary to 

 bear in mind that the deposit constituting the image is not a con- 

 tinuous film, but is more or less granular. Most of the processes 

 of redeveloping tend to increase the size of the silver grains, 

 and, if practised injudiciously upon photographs that have sub- 

 sequently to be magnified, may impair them seriously. Sulphate 

 of iron employed alone for educing a picture seems to give the 

 finest granulations, particularly if the sensitive plate, previous to 

 exposure to light, has been washed in water so as to remove 

 most of its free nitrate of silver. It requires, however, to be 

 redipped in the nitrate-of-silver bath before being treated with 

 the sulphate. The free nitrate of silver necessary to the reduc- 

 tion cannot be advantageously supplied otherwise; and the sensi- 

 tiveness would be decreased. 



No photograph that has to be magnified should be varnished. 



