On new Theorems relating to the Moon's Orbit. 27 



a saturated solution of common salt in the proportion of one fluid 

 drachm of the salt solution to four ounces of collodion. Spread 

 on the glass plate in the usual way and immerse it for one mi- 

 mute to one minute and a half in a neutral solution of nitrate of 

 silver, 30 grains to the ounce. Develope the picture with a 

 solution of sulphate of iron one scruple to the ounce of water ; 

 and finally fix with the hyposulphite of soda. A very beautiful 

 picture may also be obtained by using the developing solution 

 of sulphate of iron, of the strength of 20 or 30 grains to 4 ounces 

 of water, and adding to the hyposulphite wash strong water of 

 ammonia, in the proportion of 20 drops of the latter to 6 or 8 

 ounces of the f(Jpmer. The iron solution should be well washed 

 off previously to putting the plate in the ammonia and hyposul- 

 phite. By this process I have obtained most exquisite pictures 

 in very short spaces of time. In many cases the light parts of 

 the pictures are pure silver, forming a good mirror. 



Alkaline reaction in the bath or the collodion causes cloudi- 

 ness, which may, however, be remedied by a corresponding 

 amount of nitric acid. If a bottle of strong ammonia be left 

 open in the room where the plate is prepared, cloudiness will 

 be produced. I brought for a few minutes a dish containing a 

 solution of hyposulphite of soda, to which had been added a 

 small portion of hydrosulphuret of ammonia, into the room in 

 which I prepared a plate, and for some hours after I could not 

 get a picture without cloudiness. By thoroughly ventilating 

 the room I got rid of the annoyance. It would therefore be 

 better if the ammonia be added to the hyposulphite solution, as 

 recommended above, to keep it at a safe distance from the other 

 materials. I have also found that filtering the caustic solution 

 through the red-coloured blotting-paper is sufficient to produce 

 alkaline reaction and cloudiness. 



Parsonstown, June, 1864. 



v.. On Professor Challis's new Theorems relating to the Moon's 

 Orbit. By J. C. Adams, Esq., FM.S., Fellow of Pembroke; 



College, Cambridge. 



To the Editors of the Philosophical Magazine and Journal, 



GeNTLEMExY, 



IN the June Number of your valuable Journal, Professor Challis 

 calls attention to some circumstances connected with his 

 withdrawal of a paper, relating to the moon's motion, which he 

 had communicated to the Cambridge Philosophical Society, and 

 of the principal results of which he had given an account in your 

 Number for April (p. 278). 



Professor Challis mentions that one of the reporters, whqse 



