THE PROJECTION MICROSCOPE 189 



projections, owing to the small field-lens exhibiting so little 

 of the image, which makes it very tiresome even to find the 

 desired portion of a slide. It is, indeed, partly to their using 

 only such a very small central portion of the image, that 

 the excellence of the final image is due. The magnifying 

 power is also too high in these oculars for ordinary use, being 

 intended for a screen distance of a few feet only. For nearly 

 all demonstration purposes, all of the image that can reach 

 the end of a tube of ordinary length is quite good enough, if 

 the eye-piece is properly constructed. 



The eye-piece for projection, therefore, is one with a pro- 

 portionately larger field-lens, and with an adjustable or focus- 

 sing eye-lens, made achromatic. Such eye-pieces, of various 

 powers, will give beautiful projections with the electric 

 light ; and if of proportionately low power, which for a screen 

 distance of 25 feet should not exceed a double amplification, 

 with the lime-light. In using them, the stop of the eye-piece 

 itself is focussed sharply on the screen by the adjustable 

 achromatic eye-lens, and then the whole arrangement is 

 focussed by the fine adjustment so that the image itself also 

 appears sharp. The focussing is much quicker, or more 

 sudden, or sensitive, with an eye-piece, than when using an 

 amplifier, or the objective alone. 



The length of the body carrying the eye-piece must 

 depend upon the set of lenses used. If they are corrected for 

 the English tube, 10 inches is required. For the Continental 

 length, a 6-inch body will be required, and is more compact. 

 A draw-tube is not desirable. The body must be velvet-lined. 

 It is as well to fit it with two concave amplifiers also, which 

 will ' correct ' lenses for screen distances of about 12 feet and 

 25 feet. 



It is not well to screw objectives into separate long bodies, 

 and change these with the powers as with the short bodies ; 

 but objectives may be readily changed either by using a triple 

 nose-piece ; or what is still better, by Zeiss's now sliding 



