II. On a 'V/.s.yYY//y/w Reflector with Collected Field. 

 Dr. R. A. SAMPSON, F./t.S. 



Received Decemlwr 28, 1912, Raul February 13, 1913. 



THK great advantage enjoyed by the reflecting telescope is its equal treatment of 

 rays of all colours, and tin* geometrical defects or aberrations of its field are less 

 than those of many of the older refractors. The most serious of these defects is 

 coma, owing to which different /.ones of the object i\e d<> not place tlie light which 

 they receive from the s'une object |>oint symmetrically around-any common centre n 

 the image area, but arrange it in a radial fan or Hare, the light from the outer /.ones 

 being most diffused : besides spoiling the image this tends to neutralise, for any 

 except narrow fields, the value of extended a|)erture in the objective as a light- 

 collector. In the refractor this can be and is now always met by adjusting the 

 curves of the two lenses, for when achromatism, as far as possible, and spherical 

 aberration are allowed for, there still remains one unused datum ; in old forms this 

 was often used to make the inner curves contact curves that might be cemented 

 together if it was convenient to. do so, but it is properly employed to extinguish 

 coma. But with the reflector the case is different. In the Newtonian form there is 

 only one available surface, and when this is made a paralxiloid to cure spherical 

 aberration, nothing is left to adjust. In the Gregorian or Cassegrain forms there are 

 two curved surfaces and, theoretically, these would offer means to correct two faults. 

 An illuminating study of the possibilities of a system of two mirrors has been made 

 liy SCHWARZSCHILD in his ' Untersuchungen zur Geometrischen Optik';* I shall 

 i leal with its outcome below. Its general tenor is comprehensive and exploratory 

 rather than detailed, and it remains doubtful whether any of the forms which he 

 indicates for the reflector, at the point at which his research stops, could actually be 

 made successfully upon a scale that would show their advantages. My own purpose 

 in the present'paper is essentially a practical one. I have in mind throughout a 

 telescope of large aperture and considerable focal length, and seek to devise a 

 correction for the faults of its field which shall leave its achromatism unimpaired, 

 which can really be made and which shall effect its purpose without employing any 

 curves and angles outside those that are already known to work well. It has been 



* ' K. Gesell. d. Wissenschaften zu Gottingen, Abhandl. Math.-Phys. Claase,' Neue Folge, Bd. IV., 1905. 

 VOL. CCXIII. - A 498. E 2 Published Miparmtely, April 18, 1918. 



