SPECIAL EYE-PIECES 38 



Focus of Eye-pieces for Long Body. 



Focus of Eye-pieces for Short Body. 



Projection Eye-pieces. 2 for short and 3 for long bodies = 90 mm. or 3'54 

 inches ; 4 for short and 6 for long bodies = 45 mm. or 1*77 in. 



suited for this purpose, but a negative form is often employed, the 

 micrometer being placed inside the eye-piece in the diaphragm, i.e. 

 the focus of the eye-lens. 



In order that the micrometer may be susceptible of focus for 

 various sights, it is necessary that the eye-lens in the case of a 

 negative eye-piece, and the whole eye-piece in the case of a positive 

 one, should be mounted in a sliding tube ; and one with a spiral slot 

 will be preferable, since it makes the work of focussing both facile 

 and accurate. 



If only one micrometer eye -piece is used it should be of medium 

 power, such as H-inch focus; but it is an inexpensive and a useful 

 plan to have an additional set of lenses to screw on to the same mount, 

 so as to make the eye-piece, say, a -inch focus. 



Spectroscopic, polarising, goniometer, and binocular eye-pieces 

 are each treated under their respective subjects. 



Quekett's index eye-piece is one which has a pointer placed at 

 the diaphragm, so constructed that it can be turned in or out of the 

 field, and is used to point to the position of an object. 



A good plan, when the magnification is great, is to have a dia- 

 phragm with a small aperture to drop into the eye-piece and dimi- 

 nish the field of view. This not only makes the object to be pointed 

 out more easily accessible to the eye, but as we have by many 

 years of observation proved it aids in close observation upon minute 

 objects by cutting off a large area of light without altering the in- 

 tensity of what remains, and so makes close observation more easy. 



Diaphragms with a square aperture are fitted into eye-pieces for 

 the purpose of counting blood-corpuscles in a definite area. The 

 hole in the diaphragm must be adjusted for a definite tube length 

 and for use with a definite objective and used with 110 other. 



As it is directly associated with the eye-piece, we shall find no 

 better place to note the curious and hitherto unexplained fact, that 

 when resolving stria? or lines with oblique light the effect is much 

 strengthened by placing a Xicol's analysing prism over the eye- 

 piece. 



Testing Object-glasses. It will have been noted by the attentive 



