ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



623 



of the object (the field of vision) may be obtained by means of a small 

 source of light. This object is attained notwithstanding considerable 

 aberrations, and it is the better obtained the greater the focal length 

 of the illuminating system. A lens of 3/4 in. curvature is therefore 

 less advantageous than one of 1 in. 



The general view of English microscopists is undoubtedly in 

 favour of the superiority of an achromatic condenser over any non- 

 achromatic arrangement. With the latter, confused pencils of light 

 are produced by the spherical aberration, which seriously impair the 

 images of fine structures, whilst with the former, " the most delicate 

 objects are seen with a clearness and sharpness of detail quite 

 unknown to those microscopists whose experience has been confined 

 to the use of non-achromatic condensers." * 



Bausch's New Condenser.f — E. Bausch describes a new condenser 

 (figs. 102 and 103), similar to thaj; of Prof Abbe,, the formula upon 



Fig. 102. 



Fm. 103. 



which it is constructed being, however, a modification of that used in 

 Bausch's Immersion Illuminator. The posterior system is as large 

 as the substage-ring will allow, and will transmit and condense all the 

 rays which pass through this from the mirror. Its numerical aperture 

 is about 1 • 42. 



There are two styles of mounting, fig. 103 shows the substage 

 adapter and condenser with a swinging diaphragm ring between them. 

 This ring receives the various stops, which may be changed without 

 disturbing the condenser. Fig. 102 is intended to give the different 

 degrees of oblique illumination, from central to that of the utmost 

 possible limit. It is provided with a circular opening, 1/4 in. in 

 diameter, which may be decreased if desired, and which is caused to 

 move slowly from the centre to the edge of the mounting by turning 

 the outside milled edge. 



Both of these mountings are adapted to substages attached either 

 to the substage bar, or fixed to the bottom of the stage. The condenser 

 is also furnished with plain substage adapter only. 



Glass Frog-plate.— This (fig. 104, designer unknown) is a simpli- 

 fication of the ordinary frog-plate. The general form of the 



• Swift'.s ' The Microscope,' 188.3, p. 43. 



t TJie Microscope, iv. (1884) pp. 105-C (2 liga.). 



