200 TEAR-BOOK OF FACTS. 



justment in altitude and azimuth, was, on the 18th of July, erected 

 in the open air within the enclosures of the Royal Military Reposi- 

 tory on Woolwich Common, and immediately contiguous to all the 

 appliances of a well- furnished photographic laboratory. The object- 

 glass, 4 inches in diameter, has a sidereal focus of 77 inches, and 

 gives a representation of the sun's disc measuring, at this season of 

 the year, - 7 inch in diameter. For photographic purposes the eye- 

 piece was removed from the telescope and a small sliding-bodied 

 camera adapted to the end of the tube : it was then easy to project 

 upon the ground glass a perfectly defined image of the solar disc, 

 using the means of adjustment which the camera afforded for the 

 purpose of obtaining the best optical focus ; and in this plane the 

 prepared photographic surfaces were usually employed, for the chemi- 

 cal and visual foci had previously been ascertained to be very Dearly 

 coincident. The aperture of the object-glass was now stopped down 

 until, with a diaphragm of "25 inch, the exposure to the powerful 

 action of direct sunlight was rendered manageable ; and in order to 

 secure an easy and sufficiently rapid means of opening and closing 

 this small aperture, a card of about 6 inches square was provided, 

 having cut out from its centre a narrow slit of about an eighth of an 

 inch in diameter and nearly 2 inches in length. This, quickly moved 

 by the hand in front of the diaphragm of the telescope lens, limited 

 the period of exposure to a small fraction of a second, and besides 

 made it possible to regulate the interval of time at the taking 

 of each picture according to circumstances at tiie moment, which 

 were occasionally varying, as light fleecy clouds passed over the 

 face of the sun. Mr. Crookes, who took charge of this part of 

 the apparatus, was provided with dark glasses, to enable him, by 

 watching the sun, to select the proper moment and judge the Length 

 of time which each plate would require in its exposure ; and he 

 agrees with Mr. Spiller in preferring this system of operating to the 

 use of a fixed mechanical contrivance for opening and closing the 

 aperture of the lens. 



By proceeding in the manner indicated, the observers endeavoured 

 to restrict the photographic action to the representation of the sun's 

 disc alone, and only in the first and second plates of the series WHS 

 sufficient time allowed for the highly illuminated clouds around the sun ' 



to become imprinted in the camera ; although faintly visible bi imel hues 

 in the field of view of the telescope, their intensity was now so much 



lowered as not t.> be copied, it will be evident, also, from the fore- 

 going description, that do clockwork mechanism was required tor the 

 purpose of mtiking the apparatus follow the diurnal motion of the 

 sun. 



The glass plates on which the picture! were taken measured 2 j 

 inches by 3j inches, and wen numbered at one corner bya scratching 



diamond ; tin v wire cleaned previously and arranged in their order 



of succession. The precise momenl at which each plate received its 



ore in the camera w ■! immediately; Greenwich 



mean time being taken the same morning from the Woolwich Obser- 



. and l.i pt bj ;m ordinary good watch. 



