ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 151 



seen that a slide with a rectangular opening, if caused to move across the tube 

 in front of the sensitized plate, would in no way distort the picture, but would 

 merely stop off a portion of it, and have the effect, as it moved along, of allow- 

 ino- each part of the sun's image to act in succession on different parts of the 

 collodion, and there to record itself; but a rapidly moving object close to the 

 collodion-plate is so liable to cause a disturbance of dust, and its consequent 

 lodgement on the collodion-film, that the carrying out of the idea in this 

 manner was given up. 



The late much-lamented Director of the Observatory, Mr. Welsh, suggested 

 the plan which was ultimately adopted with success; instead of placing the 

 sliding apparatus close to the collodion-plate, he proposed that it should be 

 made & on a smaller scale and fixed as near the plane of the primary focus as 

 possible. Mr. Beckley has skilfully carried out this suggestion ; so that the 

 apparatus answers its intended object most perfectly, and the production of 

 a solar picture is now at least as easy as that of a lunar picture. The sliding 

 plate is very liaht, and moves so freely,that it does not, while in motion, disturb 

 the telescope m the slightest degree ; it is drawn downwards by means of a 

 sprint of vulcanized caoutchouc, and as soon as it is released it shoots with 

 great°rapidity across the field. The sliding plate has two apertures, one cir- 

 cular, and sufficiently large to permit of the passing of all the rays ; this is 

 used for the purpose of focusing on the screen, and also in observing con- 

 tacts of the sun's limb with the wires to be hereafter described. The second 

 aperture is square, and is fitted with a sliding piece actuated by a screw, 

 which projects beyond the telescope tube ; by means of this screw the aper- 

 ture may be completely closed or readily reduced to a slit of any required 

 width, equal to or smaller than the side of the square opening, a divided scale 

 being affixed to the screw for that purpose. - 



Previous to taking a picture the sliding plate is drawn up just so high that an 

 unperforated part of it completely shuts off the sun's image ; the plate is held 

 in this position by means of a small thread attached to it at one end, and 

 looped at the other, the loop being passed over a hook on the top of the tube. 

 When the picture is about to be taken, the retaining thread is set on fire, and 

 the rectangular aperture, as soon as the sliding plate becomes released, flashes 

 across the axis of the secondary object-glass, thus allowing the different parts 

 of the sun's image to pass through it in succession, and to depict themselves, 

 after enlargement, successively on the collodion-plate. Although the time of 

 exposure is so short as to be scarcely appreciable, yet it is necessary to regu- 

 late its duration ; and it is therefore controlled by adjusting, 1st, the strength 

 of the vulcanized caoutchouc spring; 2nd, the width of the aperture. In 

 practice, the opening is usually varied between T yh and ^th of the dia- 

 meter of the sun's focal image. 



No driving Machinery needed, except at the period oj a Jotal Eclipse— 

 It will be seen from the foregoing description that the clock-work driving 

 apparatus, described at page xxxv. of the reports for 1857, can be of no ser- 

 vice, because the photograph is taken in so small a fraction of time that no 

 appreciable distortion of the sun's image would result in the interval by allow- 

 ing the telescope to remain at rest. So rapid is the delineation of the sun s 

 image, that fragments of the limb, optically detached by the « boil of our 

 atmosphere, are frequently depicted on the collodion, completely separated 

 from the remainder of the sun's disc ; more frequently stdl from the same 

 cause the contour of the sun presents an undulating line. 



Although the clock-work driver is unnecessary for the daily work of the 

 nhotoheliogiaph, it may prove of great value on the rare occasions of a total 

 solar eclipse. It is to be hoped that it will enable the contemplated expedi- 



