476 



Grubb — On the Construction of Telescopic Ohject- Glasses. 



the plate, the images appear very perfect for a considerable distance from the 

 centre, the fact being that the coma is so widely spread, and consequently weak 

 in intensity, that it is invisible in the case of the smaller stars. On consideration, 

 however, it will be seen that such images would be useless for the purpose 

 required, viz. accurate measurements. The image of a small star would only be 

 that of its nucleus, while the image of a large star would include the coma which 

 extends in this case not symmetrically on each side of the nucleus, but almost 

 altogether on the side toward the centre ; consequently the distances of all large 

 stars from a central point would measure too small, as compared with those of 

 stars of less magnitude. 



It is not possible to obtain absolutely round images of stars anywhere on a 

 plate, except at or near the centre ; but it is evidently 

 necessary that whatever departure from a perfect circle 

 the image may be, it must be symmetrical on each side 

 of a tangential, as well as of a radial line cutting its 

 centre (see fig. 2). That is to say, if C be the centre 

 of the field, the image of the star must be symmetrical 

 on both sides of the lines AB, CD. This, then, is the 



Fig. 2. 



condition that must be satisfied in photographic objectives, as well as those 

 mentioned before as necessary to be satisfied in the case of visual objectives, 

 viz. freedom from chromatic and spherical aberrations. 



In addition to the conditions above mentioned which it is necessary should be 

 satisfied in the case of all photographic objectives, one more condition insisted on 

 in the case of the objectives for the International Survey was, that all objectives 

 should be within a very small percentage of the same focal length. 



A little consideration will show that the imposing of these two additional 

 conditions increased in a very high degree the labour in the construction of the 

 objectives. It is convenient in considering the various alterations in the form of 

 objectives to imagine the glass flexible, and capable of being bent out of its 

 normal state to any other required form. Taking the form of the ordinary visual 

 objective to start with, which with the more usually used qualities of glass is 



Fig. 3. 



Fig. i. 



Fig. 5. 



Fig. 6. 



almost a bi-convex crown with a plano-concave flint fitting as in fig. 3, it is 

 necessary, in order to eliminate the coma, that the crown be either bent forwards 

 on edge as in fig. 4, or the flint bent backwards as in fig. 5. In other words, 



