80 On Microscopical Appliances. 



and ascertaining magnifying power. It is a negative eye-piece, 

 the lenses of which are so arranged as to magnify just five 

 times under the condition of the eye-lens magnifying ten times. 

 AVithin the focus of the eye-lens is placed a circular micrometer 

 scale ruled to ^-oi^th of an inch. Upon placing another micrometer 

 ruled to ru^ths and yoVoths upon the stage, ifis plain the divisions 

 of the latter are exactly five times smaller than of the former. 

 Hence, since the eye-piece magnifies the image to the eye five 

 times, it follows that whatever power be apphed by means of the 

 objective and length of tube, the actual reading of the stage 

 T-oVoth by the eye-piece will give the actual magnifying power 

 under use. This method was employed as weU as the camera 

 lucida in estimating the magnifying advantages gained by the 

 Searchers alluded to in the December article. The extreme simph- 

 city of the instrument ls its greatest recommendation ; although its 

 modus operandi is not so very clear at first sight : but perhaps 

 the following statement may be of use towards its explanation. 



If, for instance, we view a thousandth on the stage with a " quar- 

 ter," and the Kratometer eye-piece, we shall see, without using the 

 draw-tube, that a thousandth is covered (say) by 35 • 5 divisions of 

 the eye-piece ; therefore 



Power of " Quarter " = 355 = P. 



If we draw out the tube several inches we may see that 



P = 520. 



