528^ - Transactions of the Society. 



Alternative values for h B and d' are given for the low powers 

 in Table II. to enable them to be made for the Continental or 

 No. 1 E.M.S. gauge of eye-piece. 



For the formulae upon which these eye-pieces are constructed, 

 the reader is referred to the original paper.* 



Eye-pieces on this construction are very suitable for the tele- 

 scope ; they give brilliant and sharp images upon a jet-black 

 ground free from flare or ghosts of any kind. 



Table II. contains a list of cheaper eye-pieces which satisfy 

 the first two important conditions mentioned in the description of 

 Table I. ; but as the two lenses of which they are composed are 

 plano-convex, their defining power will not be quite so sharp as 

 those in Table I. 



They will, however, give better results than the ordinary com- 

 mercial Huyghenian eye-pieces, which are often far from perfect. 

 The eye-pieces in both Tables should be "ringed," so that the 

 diaphragm may be kept level with the top of the tube of the 

 Microscope.f It should be borne in mind that the amount of over- 

 correction in compensating eye-pieces, not only differs among those 

 of different makers, but also among the compensating eye-pieces 

 Ty the same maker ; and as the same thing may be said with 

 regard to the amount of under-correction in the objectives, be they 

 achromats, semi-apochromats, or apochromats, of various makers, as 

 well as in those by the same maker, it will be seen that it is by 

 no means an uncommon occurrence to find the performance of a 

 good objective ruined by the use of a compensated eye-piece not 

 adjusted to it. 



There are some achromatic objectives which perform better with 

 compensating than with Huyghenian eye-pieces, and conversely, 

 there are apochromats which perform better with Huyghenian than 

 with compensating eye-pieces. For example, a semi-apochromat 

 of exceptional excellence, corrected for the 6 ' 3 in. tube, required a 

 tube length of 4^ in. with a Huyghenian eye-piece ; on the other 

 hand, some very fine objectives would not come into adjustment at 

 all with compensating eye-pieces, even when the tube was racked 

 out to its full extent, say 12 or 13 in. ! So in the first case one 

 would be obliged to use a compensated, and in the second a 

 Huyghenian eye-piece. 



Tube-length correction is not altogether an alternative for over- 

 correction in eye-pieces, nevertheless, if it is found that better 

 results are obtained with Huyghenian eye-pieces, of proper con- 

 struction, and tube-length correction, why use compensating eye- 

 pieces ? 



* This Journal, 1900, p. 165. 



t Powell was the first to maintain a uniform optical tube length by " ringing" 

 his eye-pieces, and he has remained alone in this excellent practice. When he 

 first began to do this is uncertain, but there is a Microscope of his of 1810 with 

 " ringed " eye-pieces. 



