376 



OBJECTIVES, EYE-PIECES, THE APEETOMETER 



a virtual image of the real image which is presented to them as the 

 object. For this purpose a combination is indispensable, but this 

 may be varied. There are ordinary and special eye-pieces. Those 

 in ordinary use separate into two divisions : (1) positive eye -pieces 

 and (2) negative eye-pieces. These are easily Distinguished ; with 

 a positive eye-piece we can obtain a virtual image of an object by 

 using it as a simple microscope, because its focus is exterior to itself. 

 This cannot be done with the negative eye-piece, because its focus is 

 within itself. 



The eye-piece in common use is negative, and is generally known 

 as Huyghens's, and sometimes as Campani's. Monconys appears to 

 have been the first (1665) to supply the field-lens to the eye-lens of 

 the microscope, and Hooke in 1665 adopted his suggestion ; but 

 how far Monconys was indebted for this to the compound eye-piece 

 attributed to Huyghens cannot now be determined. 



This instrument, as commonly used in a telescope, consists of 

 an eye-lens and a field-lens, each being plano-convex, having their 

 convex sides towards the object, their foci being in the ratio of 



FIG. 322. Huyghenian eye-piece. 



FIG. 823. Kellner eye-piece. 



3 : 1, and the distance between them being equal to half the 

 sum of their focal lengths, a diaphragm being placed in the 

 focus of the eye-lens. In a microscope a different ratio and lens 

 distance is employed, the fact being that different tube lengths 

 require different formulae. The general form of a Huyghenian eye- 

 piece is shown in longitudinal section in fig. 322. This makes a 

 very convenient form of eye-piece of 5 and 10 magnifying power ; 

 but when the power much exceeds this last amount the eye -lens 

 becomes of deep curvature and short focus, so that the eye must be 

 placed uncomfortably near the eye-lens. This, however, is its chief 

 defect, and it may fairly be considered the best ordinary eye-piece. 



Another negative eye-piece is that known as the Keliner, or 

 orthoscopic. This consists of a bi-coiivex field-glass, and an achromatic 

 doublet meniscus (bi-convex and bi-concave) eye-lens. A vertical 

 section of one so constructed is seen in fig. 323. These eye-pieces 

 usually magnify ten times, and the advantage they are supposed to 

 give consists in a large field of view ; but they are not good in practice 

 for this very reason ; they take in a field of view greater than the 



