Lieut-General Strachey and Mr Whipple. [Apr. 



of \\ hii-li the azimuth and altitude of any point in the cloud pictni 

 could be deduced from those of the intersection of the cross lines. 



Arrangements were made for erecting these cameras at the ex- 

 tremities of a base of known length (800 yards), between which 

 electrical communication was established. 



Spring shutters were placed over the lenses, which could be Ml 

 rated and again closed, at the will of the observer, by the passage 

 an electric current, so as to expose the plates for any desired inter 

 of time. 



Captain Abney also, after numerous trials, devised a suitable for 

 mula for an emulsion for coating the plates, as special precautiot 

 were found to be necessary in order to obtain good cloud photograpl 



Captain Abney thus describes the photographic process he pi 

 posed : " My attention has been once more directed to the 

 photographic process to employ for the delineation of the clouds, 

 certain inconvenience having attached to the use of collodion-emulsic 

 which at first I had not foreseen. I had then recourse to gelat 

 plates, but the manner in which they are ordinarily prepared indue 

 a sensitiveness which becomes unmanageable, even when a diaphrag 

 with a small aperture is used in the lenses. The great desideratt 

 in the plates appears to be that a small variation in the intensity 

 the light proceeding from the sky or cloud shall produce a 

 contrast in the intensity of the developed image. A very rapid plat 

 does not answer for this purpose ; hence I tried several modific 

 tions. The process which at present has given the best results is 

 follows : 



" 150 grains of bromide of ammonium and 10 grains of iodide of 

 potassium are dissolved in 3 oz. of water, to which 80 grains of 

 Nelson's No. 1 photographic gelatine and 80 grains of Coignet's 

 gelatine have been added. This is dissolved by the aid of heat, and 

 200 grains of silver nitrate dissolved in 1^ oz. of water are added. 

 The whole is warmed to 100 F. for five minutes, and allowed to set 

 after being poured out in a flat dish. The emulsion thus produced is 

 washed (in the usual manner) from the soluble salts, and is the 

 re-melted and plates coated and dried, as is customary in the gelat 

 process. 



" This formula gives very constant results, and great contrasts 

 image arc obtained by careful development." 



The years 1881 to 1884 were passed in working out the det 

 of the arrangements above described, and in 1885, after numeror 

 ])reliminary trials, it was resolved to erect the two cameras at the 

 Kew Observatory. One was placed on the roof of the Observator 

 building, and the other on a stand in the Old Deer Park, 800 ya 

 from the other, on the road leading to the Observatory from Rich- 

 mond ; and a telegraph cable carrying two insulated copper wires of 





