OIUECTIYES, EYE-PIECES, THE APERTOMETEK 



FIG. : 



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

 bi-convex eve lens and no field-glass. The definition witS this must 

 liave lieen most imperfect; the addition of a field-lens, though it 

 were a l>i-cniive.\ not in the correct ratio of focus nor the theo- 

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



In this wav mallei's rested, however, until the theoretically 

 perfect Huvghenian form was devised. Object-glasses have been 

 used ,-is eye pieces, a lid all forms of lotips 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 110 

 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 Steiii- 

 heil triple loups ; a section of one of these is 

 seen in tig. H25. This combination also forms 

 one of the best lenses for projection purposes 

 e\ er const i-ucted. But a positive eye-piece was devised by Ramsden, 

 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 

 ape-chromatic 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 maybe 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 Huvghenian 

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

 ye-lens. 



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

 Abbe. These are : 



1 . Searcher eye-pieces. 

 1. Working ,, 

 .'{. 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 

 lield lens and a doublet eye lens. 



Tin' n-nrL'/iiij i'orinsare both posil ive and negative. The eye-piece 

 lor the long tube has a triplet eye-lens; but the remainder, vi/,. S. 

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

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

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

 It may lie convenient to have the K a negative like the 4. but with 

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

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

 a t riple wit h a single piano convex over it : t he diaphragm is. of course, 

 exterior to the lens (fig. :!2li). With these the definition is of the 

 &nes1 qualify throughout the field, which has been reduced to about 

 i) inches They present the admirable condition that with the deeper 



1 Chapter 1. p. 88. 



