206 repoiit — 1859. 



light, bull's-eye, and slit may be pushed vertically (not horizontally as in the 

 others) a little to one side of the central line of the lens, so that the dots may 

 assume a different position on the sensitive paper. 



The distance between the slit and the lens is 1Y'6 inches, that between 

 the lens and the mirror is 8-1 inches, while the distance between the mirror 

 and the cylinder is 6 feet. 



Hence the focal length of the lens for actinic parallel rays is about li'i 



inches. 



4. Registering Cylinder and Clockwork. 



These are exhibited in Plate 4*. figs. 8 & 9. The cylinders are each 

 6-i- inches long, and 6 inches in diameter. They consist of brass silvered 

 over. The method of connecting them with the clockwork was devised 

 and executed by Mr. Beckley. The toothed wheel k is driven by the clock- 

 work, and drives the two pinions I. These pinions, when in gear, drive the 

 two horizontal cylinders by means of teeth attached to the circumference of 

 the latter. Two radial arms, to which the pinions I are attached, enable 

 these to be put out of gear when it is necessary to remove the cylinders. 

 The position of the pinions in this case is indicated in the figure by dotted 

 lines. The vertical cylinder has a toothed rim attached to its lower extremity, 

 which is driven by the crown wheel m. By removing a screw, the cylinder 

 may, when necessary, be detached from its toothed rim, leaving the latter 

 behind. 



Section III. Description of the Photographic Process. 



The process employed is that known as the waxed-paper process, and is 

 thus described by Mr. Crookes. 

 Description of the Wax -paper Photographic Process employed for the Photo- 



meleorographic Registrations at the Radcliffe Observatory. ByW. Crookes, 



Esq. 



1. Before attempting to select from the numerous Photographic processes 

 the one best adapted to the requirements of Meteorology, it was necessary 

 to take into consideration a number of circumstances comparatively unim- 

 portant in ordinary operations. 



To be of any value, the records must go on unceasingly and continu- 

 ously : 



First. Therefore, the process adopted must be one combining sharpness of 

 definition, with extreme sensitiveness, in order to mark accurately the minute 

 and oftentimes sudden variations of the instruments. 



Second. To avoid all hurry and confusion, it is of the utmost importance 

 that the prepared paper or other medium be of a kind capable of retaining 

 its sensitiveness for several days. 



Third. The contraction which paper undergoes during the numerous 

 operations to which it is subject in most processes (in general rather an ad- 

 vantage than otherwise), is here a serious objection ; for this reason, the 

 experiment first tried, of transferring to paper the image received on col- 

 lodion preserved. sensitive by the nitrate of magnesia process, was a failure. 



Fourth. Strong contrast of light and shade, and absence of half-tint, un- 

 fortunately so common amongst ordinary photographic pictures, is in this 

 case no objection. 



Fifth. It is essential to preserve the original results in an accessible form ; 

 and for this reason, the Daguerreotype process, admirably as it seems to 

 answer other requisites, is obviously not the one best suited to our purpose. 

 Lastly, the whole operation should, if possible, be so easily reducible to 



