ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 133 



let it be assumed that the telescope is mounted on an axis parallel with the 

 earth's axis, and provided with a driving apparatus, capable of carrying the 

 telescope round in the direction of the star's apparent path so equably, that, 

 if viewed with a micrometer eye-piece, the image of the star would remain 

 always in contact with one of the wires of the eye-piece. The photographic 

 picture of a star, obtained by a telescope under these conditions after some 

 seconds' exposure, is not one single clear disc or point, but a conglomeration 

 of points, extending over a greater or less area, according as the atmosphere 

 has during the interval produced more or less flickering. 



If a mere speck, like a fixed star, acquires comparatively large dimensions 

 on a sensitized plate in consequence of atmospheric disturbances, every 

 optical point in an image of other celestial objects must, from the same 

 cause, occupy a space of greater dimensions than it would if no disturbing 

 influences existed. When the telescope is employed optically, the mind can 

 make out the proper figure of the object, although its image dances before 

 the eye several times in a second, and is able to select for remembrance 

 only the states of most perfect definition ; on the other hand, a photographic 

 plate registers all the disturbances. The photographic picture will conse- 

 quently never be so perfect as the optical image with the same telescope, 

 until we can produce photographs of celestial objects instantaneously : we are 

 still a long way from this desirable end. 



Relative Advantages of Reflecting and Refracting Telescopes for Photography. 

 With refracting telescopes, the photographic focus of a point of light 

 occupies a larger area than with reflectors ; this is especially the case with 

 Astronomical Telescopes, because they are corrected so as to produce the best 

 optical image, and the outstanding chemical rays are dispersed around the 

 luminous focus*. The reflecting telescope has, therefore, considerable ad- 

 vantage over, the refracting telescope for celestial photography, on account 

 of all rays coming to focus in the same plane ; hence, the focus having been 

 adjusted for the luminous image, it is correct for the chemical image, 

 and has not to be disturbed, as with a refractor. In the telescope employed 

 by Professor Phillips, of 6£ inches aperture and 11 feet focal length, the 

 actinic focus was found to be 075 inch beyond the visual focus ; and in the 

 Liverpool Equatorial of 12| feet focal length the actinic focus was 0*8 inch 

 beyond the visual focus. With my telescope the focusing is critically 

 effected with the aid of a magnifier, the image being received on a piece of 

 ground glass placed temporarily in the actual slide destined to contain the 

 sensitized plate ; a second piece of ground glass fixed in a frame is put into 

 the camera just previous to each operation, for the purpose of placing the 

 telescope in position ; but the focusing is always effected in the manner de- 

 scribed, for the goodness of the picture depends greatly on the accuracy of 

 this adjustment. I attribute much of my success to the employment of a re- 

 flector", while my fellow-labourers in the same field have used refractors. 



Actual Process employed at the Cranford Observatory. 

 With the view of facilitating the labours of others desirous of entering 

 the field of photography, I will now describe, with all necessary minuteness, 

 the process finally adopted after many trials and failures ; I would remark 

 at the same time that it is quite impossible to give such directions as will 

 enable another operator to ensure perfect results, as this can only be attained 

 by perseverance, long practice, and a strong determination to overcome 

 obstacle after obstacle as it arises, — therefore, no one need hope For 



* Refracting telescopes can be specially corrected for the chemical focus in the same way 

 as Camera lenses. 



