42 DR. R. A. SAMPSON ON A 



w have <J,H = -4, wlii-li may be compared with (5 3 H = -1 for a Newtonian, 

 lut since the aperture ratio aff' is diminished in the ratio 1 : 4 by the increase of 

 effective focal length, the radii of focal circles at all distances from the centre of 

 the field will have the same angular amount that they had in the Newtonian form, 

 neitluT more nor less. There remains then only the above-found curvature of the 

 field to notice. Taking as a convenient mark a distance 34''3 from the centre of the 

 field, namely where /3 in the formulae of p. 30 equals one-hundredth, we should have 

 at this point the field curved back from the plane through the principal focus by 

 more than one inch. In spite of this pronounced curvature, exquisite photographs of 

 the Moon, as well as of small objects like Mars, have been obtained with this 

 telescope in Cassegrain form. The photograph of the Moon (R.A.S. photographs, 

 No. 214) appears to me second only to the Yerkes photographs with the 40-inch 

 refractor and colour screen ; but technically it would lie more instructive to examine 

 a photograph of a wide field of stars. 



It is worth while to demonstrate that curvature of the field cannot be removed by 

 replacing the second mirror by a set of lenses in contact, used as a reverser, as 

 explained on p. 37. By such a replacement we introduce the quantity $ which, for a 

 given focal length of the reverser, is adaptable by throwing different proportions of 

 the deviation of the rays upon the lens system and silvered surface respectively. 



Then using the formulae (4) of p. 32, in which we may put hk = -1,1= l,k now 

 referring to the great mirror and * to the reverser x 



where, if Jjy, ...-* refer to the reverser at its surface, 



by (11), p. 37, 



Thug 



m 

 Eliminate ft,*...) by forming 



