f 

 110 Hall^ on a New Form of Condenser. 



it to the under plate of the stage of the microscope, and an 

 inner sliding tube within that to carry the lenses, and a 

 diaphragm with perforations for a polarizing prism, spot for 

 dark-ground ilhiniination, and shutter for oblique light. In 

 the more expensive instrument the focus is obtained by a 

 rack-and-pinion adjustment, and the upper part of the tube is 

 pierced so as to admit of a frame, having two central stops, to 

 sHde closely beneath the .field glass, thereby giving a more 

 intensely dark ground than can be got with the stops at a 

 greater distance from the lenses, and at the same time per- 

 mitting the polariscope to be used in conjunction with the 

 spots. 



The large diaphragm has also two smaller ones revolving 

 upon it — one pierced with a series of holes, gradually increas- 

 ing in diameter, and the other with three perforations, one 

 ojDcn, two containing selenite films so arranged as to rotate 

 behind the polarizer. Lastly, there is a revolving cap to carry 

 stops for the examination of test objects, the stops being made 

 removable at the will of the operator. The various j^arts 

 requiring it are centered by spring catches. 



The special value of this condenser is considered to be — 



1. It can be used with marked advantage with objectives 

 from 2 inch to |^th inch ; with my Powell and Lealand's ith 

 and D eye-piece I have easily checked the dots on P. angu- 

 latum. 



2. The remarkable daylight softness produced by the tinted 

 field lens when used with artificial light, also dispensing with 

 the necessity of blue lamp chimneys. 



o. It is a very effective spot lens, and dark-ground illumi- 

 nator, with polarized light. 



4. An almost indispensable requisite for polarized light 

 when using high powers with the object mounted in fluid. 



5. And not least important, the ease and rapidity with 

 which the changes from ordinary to oblique and plain or 

 coloured polarized light, with the other combinations I have 

 named, can be made. 



That you may have the opportunity of examining the 

 instrument, and judging of its worth for yourselves, I am 

 desired by Mr. Swift to ask the Society's acceptance of one 

 in its complete form, with polarizer and paraboloid, and 

 adapted to the microscopes made by him for the Society. 



