96 E. M. NELSON ON MICROSCOPE CONSTRUCTION 



in a field of view is $ inch, therefore it is necessary for the 

 condenser to focus upon the stage an image of the flat of the 

 flame $ of an inch wide. 



The condenser has a low aperture of N.A. 0*14, but large enough 

 for the objectives for which it is intended to be used. 



NAVICULA RHOMBOIDES AND ALLIED FORMS.* 



(Addendum.) 



By Edward M. Nelson, F.R.M.S. 

 {Bead November 2M7i, 1912.) 



With reference to the question, " What was the Amician Test?" 

 quite accidentally I recently came across a notice to the effect 

 that the test used by the Jurors at the International Exhibition 

 (London, 1862) was the Navicula ?'homboides under the name of 

 JV. affinis. This of course clears up all the difficulty. This 

 N. rhomboides would have been of the kind termed the 

 " English " rhomboides in my paper, and would have had 72 to 

 73 striae in 0*001 inch. 



ON MICROSCOPE CONSTRUCTION AND THE SIDE 

 SCREW FINE ADJUSTMENT. 



By Edward M. Nelson, F.R.M.S. 



(Bead November 26th, 1912.) 



There is one point which has been overlooked with respect to 

 the evolution of the microscope. It is thought that the modern 

 plan of placing the coarse adjustment slide and the body upon the 

 fine adjustment was the invention of Zentmayer (1876), and that 

 it first appeared in this country in the Ross-Zen t may er model. 

 This however is not the case, for Powell in 1841 invented this 

 plan, as well as that of the side pinion fine adjustment, now so 

 much in vogue. 



In the frontispiece of Cooper's Microscopiccd Joumcd an illus- 

 tration of this model will be found, f Some years ago Mr. T. 



* Journal Q.M.C., Ser. 2. vol. ii. p. 93. 



f This was the first microscope Powell introduced after Lealand had 

 joined the firm (vide Journal B.M.S., 1900, p. 287, fig. 78). 



