525 



XIII. — Eye-pieces for the Microscope. 

 By Edward M. Nelson. 



{Read June IWi, 1907.) 



As several friends have asked me to provide the numerical values 

 for some of the eye-pieces described in my Presidential Address for 

 the year 1900, I thought it would probably save further trouble if 

 a list were published. In my own work the eye-pieces in Table I. 

 have quite superseded those of the compensating form. There is 

 no reason why these eye-pieces should not be produced at a price 

 only slightly in excess of the ordinary Huyghenian, as they are 

 composed of only two bi-convex lenses. In these eye-pieces in 

 Table I., the refractions are equally divided between the two 

 lenses, the equation for achromatism (given by Coddington, in his 

 " System of Optics," at the foot of p. 261, as well as by other 

 writers) is also satisfied, and the direct pencil is equally refracted 

 by both surfaces of the field-lens. 



In the following Tables s is the radius of the surface of the eye- 

 lens next the eye, r that of the other surface, and h is the diameter 

 of the eye-lens. 



- - S, E, and B have a similar significance with regard to the field- 

 lens ; d' is the distance of the lenses apart, measured from their 

 surfaces (which is 3^ p.c. less than d in the original formula, where 

 the distance is that between the Gauss points) and h is the diameter 

 of the hole in the diaphragm. 



h and h = cf ; c = '575 for a 6 inch field. 



= for long tube, and for short tube. 



m m 



These values of B require to be increased to allow for the 



edging and mono ting of the lens. In the Tables, B for the long 



6'9 7*19 



tube was made — for powers of 12 and under ; for powers 



m m 



15 to 25; and for powers 30 to 40. 



m 



The corresponding values for the short tube were 



4-35 4-53 ,4-71 



• ; and . 



m m m 



