254 Dr J. B. Pearson, On the construction [Oct. 30, 



Mr J. R. Green, Trinity College, was balloted for and duly 

 elected an associate of the Society. 



Mr Glaisher having taken the Chair, the following communica- 

 tions were made to the Society: 



(1) On the construction of a negative eye-piece. By Dr J. B. 

 Pearson. 



I wish to be allowed to offer a few remarks on the construction 

 of the negative astronomical eye-piece : only as designed for the 

 use of abnormally short-sighted or long-sighted persons. For good 

 average sis'ht, I have no doubt that the existing construction is all 

 that is to be desired. 



Sir E. Beckett (Astronomy without mathematics, p. 301) says: 

 "All eye-pieces are made adjustable for different eyes by the eye- 

 glass sliding nearer to the field-glass for short-sighted eyes, 

 Avhich require the rays of each pencil to diverge a little instead of 

 being parallel." It is true this is the case with the sextant-circle 

 telescopes which I use, the two German ones having positive eye- 

 pieces, and the French a negative one: but it is not so with the 

 common astronomical negative eye-pieces, such as are sold in 

 England. Now with powers up to say 70, and an aperture of 

 about 3 inches, I have found I got very good results by focussing 

 in the usual way; but with a power of 120, I found I could get no 

 satisfactory definition ; and as a fault, obviously the same, came 

 out with different object-glasses, I was led to examine the con- 

 struction of the eye-piece to see whether part at any rate of the 

 fault might be there. 



Theoretically, and in practice, for perfect eyes, the focus and 

 diaphragm or stop of a negative eye-piece are placed between the 

 two lenses : but if a short-sighted person adapts the focus of the 

 telescope to his own eye, he moves the whole eye-piece by means 

 of the large screw. This really means that he pushes forward the 

 field-glass, i.e. the lens nearest the object-glass, so that it stands 

 nearer to the object-glass : by this means the field-glass intercepts 

 portions of the pencils coming through the object-glass, which on 

 an average are more nearly parallel to their axis : the focus of the 

 rays passing through the object-glass and field-glass is thus thrown 

 farther from the field-glass than it was before; and, as the distance 

 between the field- and eye-glass is unchanged, also nearer to the 

 eye-glass : they thus diverge after passing through the eye-glass, 

 and are consequently fit for vision by a short-sighted eye. 



This process answers very well in ordinary cases and with 

 moderate magnifying powers, but if extreme precision is needed, 

 it seems to me to be defective. The field-glass foreshortens slightly 

 the focal length of the object-glass, but its special purpose is to 



