Photography at Greenwich Observatory . 23 



The thermometers are mercurial, and have large "bores 

 nearly half an inch in diameter. Fine wires are placed at 

 every degree across a plate which covers the tube, and has a 

 slit in front of the mercury column, with thicker wires at every 

 10°, and extra ones at 32°, 52°, and 72°. 



The light of a jet of naphthalized gas is condensed by a 

 cylindrical lens upon the thermometer tube, and as the mercury 

 rises or falls, it obscures or uncovers the sensitive paper, and 

 leaves a broad photographic trace on each sheet, which shows 

 the height of the thermometer, and the exact degree can be 

 ascertained from the numbers indicated by the spaces of light 

 produced by the intercepting wires. This cylinder is larger 

 than the others, being 13^ inches long and 19 inches in cir- 

 cumference ; it rotates once in 48 hours, and requires no 

 base-line. 



The preparation of the sensitive paper is a point of great 

 interest, and after many comparisons with other processes, the 

 following one, which has been very little altered from the com- 

 mencement of the photographic registration, is still adhered 

 to. It was a matter of considerable difficulty to devise a 

 system which, while the paper should be so sensitive as to be 

 affected by artificial light, should also retain this quality for 

 one or two days, and then allow the hitherto invisible trace to 

 be developed with an equal amount of intensity. We give the- 

 process in Mr. Grlaisher's own words, slightly abridged : — 



" The paper is made by Hollingworth, and is a strong one,, 

 of even texture. 



c< First Operation. — Prehminary preparation of the paper. 



" 1. Sixteen grains of iodide of potassium are dissolved in 

 one ounce of distilled water. 



" 2. Twenty-four grains of bromide of potassium are dis- 

 solved in one ounce of distilled water. 



' ' 3 . When the crystals are dissolved, the two solutions are 

 mixed together, forming the iodizing solution. The mixture 

 will keep any length of time. 



" Immediately before use, it is filtered through filtering 

 paper. 



"A quantity of paper, sufficient for the consumption of 

 several weeks, is treated in the following manner, sheet after 

 sheet : — 



" The sheet of paper is pinned on a board, and a sufficient 

 quantity (about fifty minims for a sheet of paper fifteen inches 

 long and nine and a-half inches broad) of the iodized solution 

 is applied by pouring it upon the paper in front of a glass rod, 

 which is then moved to and fro till the whole surface is uni- 

 formly wetted by the solution. 



The paper thus prepared is allowed to remain in a 



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