[ 269 ] 



^be Elcmente of fllMcroecopv^- 



I. — The Instrument — {continued). 

 By C. E. Bousfield, L.R.C.P., M.R.CS. 



THERE is another method of employing the concave mirror, 

 which is extremely valuable when intense illumination of 

 moderate angle is required. It consists in placing the 

 mirror at its principal focal distance from the object, and inter- 

 posing between the lamp and the mirror the ordinary bull's-eye 

 condenser. The bull's-eye is so placed that the flame is in its 

 principal focus, and by this means parallel rays are thrown on to 

 the mirror and are brought to a focus upon the object. To 

 obtain the full advantage of this method, the lamp should be 

 placed by the side of the stage of the microscope, and upon a 

 level with it. If this is not done, much light is lost, in conse- 

 quence of the mirror being placed at a considerable angle with 

 the stage ; whereas by the suggested arrangement, the light returns 

 almost along the same path by which it travelled to the mirror, 

 and the latter, being turned nearly full on to the stage, offers the 

 greatest possible surface for reflection and condensation. The 

 concave mirror has not hitherto received the attention it deserves 

 in its applications to the microscope, and it will well repay any 

 time spent in acquiring a thorough knowledge of its properties. 

 By its use under proper conditions results may be obtained which 

 will challenge comparison with those got from a low-angled, sub- 

 stage condenser. When, however, the concave mirror forms a 

 large part of the surface of a sphere, the rays are no longer 

 brought, even approximately, to a point, and it is consequently of 

 very inferior utility for objectives higher than a low-angled half- 

 inch to a simple substage condenser. Of these there are many, 

 varying in price from fifteen shillings to as many pounds. For the 

 beginner we would recommend a simple form, constructed by 

 Baker and sold at the price first mentioned. We cannot, how- 

 ever, refrain from referring to the splendid condenser made by 

 Zeiss, which can be purchased complete, wnth iris diaphragm, for 

 ;2^3 I OS. The addition of a substage condenser renders the 



