JOUENAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



OCTOBEK, 1909. 



TRANSACTIONS OF THE SOCIETY. 



XIV. — On the Measurement of Very Minute Microscopical Objects. 



By Edward M. Nelson. 



{Read February 17, 1909.) 



It will be admitted that the methods of measurement of very 

 minute microscopical objects, viz. those too small to span with the 

 wires of a screw micrometer, are in a very chaotic state. Two 

 methods appear to be in vogue : the first, is by making an 

 accurate drawing by a camera and then by measuring the 

 drawing ; the second, is by making a photomicrograph and by 

 measuring the image as before. In criticising these methods let 

 the second case be taken first. The disadvantage of the photo- 

 graphic method is truly radical, for it is not always possible to 

 photograph very minute microscopical objects. It is the blun- 

 dering incompetent microscopist who discovers minute points by 

 the photographic method ; for there is not a single known micro- 

 scopic object that has been discovered by photography, which 

 cannot easily be seen visually through the Microscope in the 

 ordinary way ; but there are many objects that have been seen 

 visually, which the most expert photomicrographers have been 

 unable to image on the plate. 



In my own experience only one object has been discovered 

 photographically, but afterwards it was readily seen through the 

 Microscope. 



The disadvantage of the first method is, that the accuracy of 

 the result depends entirely upon the skill of the delineator. 



The new method, about to be described, though not free from 

 objections, is probably better than either of the two plans just 

 mentioned ; for example, personal ecpaation enters into the scheme • 

 Oct 20th, 1909 2 p 



