THE MODERN REFLECTING TELESCOPE. 51 



which acts on a set of differential gears in the shaftiafj connectinsr the driving-clock 

 and the driving-worm. This device is used on the 2-foot reflector and on the 

 30-inch cadostat, and is extremely simple and effective. 



Slow-motion in declination is given by means of a small reversible motor 

 which acts on the long sector attached to the upper trunnion shown in Fig. 22, 

 Plate XTL 



In concluding this necessarily brief and incomplete description of a modern 

 reflector mounting, attention should be called to an attachment which is absolutely 

 indispensable for the best I'esults in direct photography of all celestial objects 

 requiring long exposure. I refer to the double-slide plate-carrier, by means of 

 which hiind-guiding or coriecting for the incessant small irregular movements of 

 the image, which are nearly always visible in large telescopes, can lie done incom- 

 parably more accuratel}' and quickly than by any other means now known. This 

 device is due to Dr. Common, who described it in Monthly Notices, Vol. 49, p. 297. 

 In 1900 the writer designed and constructed a small attachment of this kind for 

 use with the 40-inch refractor and the 2-foot i-eflector ; this attachment and its use 

 are described in the Astro})hysical Journal for December 1900, \). 355. 



The photograph of the central parts of the Andromeda Nebula (Plate i), was 

 made by the writer with this small plate-carrier attached to the 2-foot reflector. 

 The exposure time in this case was iow hours. The images of the fainter stais on 

 the original negative are only 2 seconds of ai'c in diametei'; stars are shown which 

 are more than a magnitude fainter than the faintest stars which can be detected 

 visually with the 40-iuch I'ef ractor ; intricate structure and details are shown in 

 the nebulosity, which are entirely invisible with the 40-inch refractor and all other 

 visual instruments, and which have never been photographed before. When it is 

 remembei'ed that the focal length of the 2-foot reflectoi' is only 93 inches, and that 

 the aperture was in this case reduced to 18 inches, in ordei- to secure a larger field 

 than is \vell covered when the full aperture is used, some idea can be gained of 

 the lesults which could now be obtained in celestial photography with a modern 

 reflecting telescope which would comiiare in size, cost, and refinement of woi'kman- 

 ship with the gi-eat modern refractors. 



In Plate xiii is shown the large doiihle-slide plate-carrier, taking 8 x 10 inch 

 plates, which was constructed from the writer's designs in 1901, for use with the 

 40-inch refractor; the plate-cari'ier is here shown connected to the eye-end of the 

 o-reat telescope." A description of this attachment, togethei- with some photographs 

 obtained with it, will be found in the Publication.'i of the Yerhs Ohsei'vatory, 

 Vol. II, p. 389. 



