170 



Prof. C. Pritcliard. On the Capacities of two [May 3, 



II. The relative amounts of light reflected by each. 

 III. Their relative capacities in respect of distortion in the figure 



of the stellar images, and the optical distortion of the field. 

 IY. Their photographic capacity in respect of the faintest stars 



impressed on plates, with exposures of given duration. 



I. TJie General Character of the Stellar Images impressed. 



It was originally proposed to employ the same sized plate, viz., 

 4 inches square, for both mirrors, and thus in the -|-|-inch mirror have 

 the opportunity of examining a field of about nine square degrees ; 

 but it was found impossible, inasmuch as the images, even towards 

 the centre of the plate, were found to be impressed with a white centre. 

 To a certain extent, these malformations were predicted in a paper by 

 General Tennant in the ' Monthly Notices of the Royal Astronomical 

 Society.' 



This phenomenon necessitated the abandonment of so large a plate 

 with its circular carrier of seven and a half inches diameter, for a 

 smaller plate and smaller carrier having an angular field of 1° 56' or 

 nearly four square degrees. With this plate the images became 

 round in the centre, and continued so to ■ a distance of about 40' 

 from the centre. Then they became decidedly elliptical, having their 

 extremities remote from the centre fainter than the opposite extremi- 

 ties. At the edge of the plate, the figure of the star on the side 

 remote from the centre appeared to be not closed at all, but presented 

 the appearance of a fan. I have, however, not observed the focal 

 lines at right angles to each other, as seen and described by the 

 Astronomer Royal. In the ^^-inch mirror and 4-inch plate, which 

 presents also a field of nearly four square degrees, the phenomena 

 here described are generally very much less pronounced, and commence 

 at a greater distance from the centre. 



The conclusions which I feel disposed to draw from the foregoing 

 remarks, are the general unsuitability of mirrors of short focal 

 length, and the impossibility of obtaining a large angular field in 

 such mirrors, of a character serviceable for charting the heavens by 

 means of photography. How far this difficulty may be obviated in 

 refractors suitably corrected, and of comparatively short focal length, 

 it is beyond my experience to indicate. Before instituting this 

 trial, 1 had some hope, that with so simple an optical appliance 

 as a mirror, a much larger available field might have been practically 

 secured than has proved to be the case. I apprehend, however, that 

 in point of light, that is, having regard alone to the faintness of the 

 stars which, cceteris paribus, can be photographed, the advantage is 

 practically on the side of the reflector. 



Another point of some importance in the character of the images 



