26 THE MICROSCOPIST. 



Eyepieces. The eye-piece usually employed is the Huy- 

 genian, or negative eye-piece (Fig. 4). This is composed 

 of two plano-convex lenses, with their plane sides next 

 the eye. Their focal lengths are as 1 to 3, and their 

 distance apart half the sum of their focal distances. 

 Several of these, having different magnifying powers, are 

 supplied with good microscopes. It is best to use a weak 

 eye-piece, increasing the power of the instrument by 

 stronger objectives when necessary. Kellner's eye-piece 

 has the lens next the eye made achromatic. The peri- 

 scopic eye-piece of some of the German opticians has both 

 lenses double convex. This gives a larger field of view 

 with some loss of accurate definition. For high powers, 

 I have used a strong meniscus in place of the lower lens 

 in the Huygenian eye-piece. Dr. Royston Pigott has 

 suggested improvements in eye-pieces by using an inter- 

 mediate Huygenian combination, reversed, between the 

 objective and ordinary eye-piece. This gains power, but 

 somewhat sacrifices definition. Still better, he has pro- 

 posed an aplanatic combination, consisting of a pair of 

 slightly overcorrected achromatic lenses, mounted mid- 

 way between a low eye-piece and the objective. This 

 has a separating adjustment so as to traverse two or 

 three inches. The focal length of the combination varies 

 from one and a half to three-fourths of an inch. The 

 future improvement of the microscope must be looked for 

 in this direction, since opticians seem to have approached 

 the limit of perfection in high power objectives, some of 

 which have been made equivalent to g'gth or T ^th of an 

 inch focal length. As an amplifier, I have used a double 

 concave lens of an inch in diameter and a virtual focus of 

 one and a half inches between the object-glass and the 

 eye-piece. If the object-glass be a good one, this will 

 permit the use of a very strong eye-piece with little loss 

 of defining power, and greatly increase the apparent size 

 of the object. 



