378 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



eye-pieces were all positive ; that is to say, they consisted of a single 

 bi-convex eye-lens and no field-glass. The definition with this must 

 have been most imperfect ; the addition of a field-lens, though it 

 were a bi-convex not in the correct ratio of focus nor the theo- 

 retically best distance, must have been considered a great advance. 



In this way matters rested, however, until the theoretically 

 perfect Huyghenian form was devised. Object-glasses have been 

 used as eye-pieces, and all forms of loups or simple microscopic 

 lenses have been employed for the same pur- 

 pose. Solid eye-pieces have also been used 

 both in England and America, but with no 

 results that surpassed a well-made Huy- 

 ghenian combination ; but the best form of all 

 of the combinations which have been tried by 

 us as positive single eye-pieces are the Stein- 

 heil triple loups ; a section of one of these is 

 FIG 325. seen i n % 325- This combination also forms 



one of the best lenses for projection purposes 



ever constructed. But a positive eye-piece was devised by Bamsden, 

 consisting of two plano-convex lenses of equal foci ; the distance 

 being equal to two- thirds the focal length of one. The diaphragm 

 was of course exterior. 



Abbe's Compensating Eye-pieces. We have already given a 

 general description of the nature and action, in connection with the 

 apochromatic objectives, of this form of eye-piece. 1 In the section 

 above on objectives we have referred to the fact that these eye-pieces 

 are over-corrected ; this may be easily seen by observing the colour 

 at the edge of the diaphragm, which is an orange-yellow. If we 

 compare this with the colour in the same position with a Huyghenian 

 eye-piece, this will be blue, being seen through the simple unconnected 

 eye-lens. 



There are three kinds of compensating eye-piece as designed by 

 Abbe. These are: 



1. Searcher eye-pieces. 



2. Working ,, 



3. Projection ,, 



1 . The searcher forms are negatives of very low power, intended 

 only for the purpose of finding an object ; they consist of a single 

 field -lens and a doublet eye-lens. 



The working forms are both positive and negative. The eye-piece 

 for the long tube has a triplet eye-lens ; but the remainder, viz. 8, 

 12, 18, and 27, when first introduced, were all positive. The 8 was 

 subsequently, however, changed for a negative. Having used both, 

 we are glad to learn that it is made now both positive and negative. 

 It may be convenient to have the 8 a negative like the 4, but with 

 regard to the 12, 18, 27 it is important that they should be positives. 



These positive forms are on a totally new plan, being composed of 

 a triple with a single plano-convex over it ; the diaphragm is, of course, 

 exterior to the lens (fig. 326). With these the definition is of the 

 finest quality throughout the field, which has been reduced to about 

 6 inches. They present the admirable condition that with the deeper 

 1 Chapter I. p. 83. 



