168 THE MICROSCOPE. 



lamp; and the eye-piece and object-glass suitable for 

 examining the object may be employed, and the focus 

 adjusted accordingly. The conical diaphragm with its 

 various apertures may now be rotated, until that quality 

 of illumination is obtained which gives a cool, distinct, 

 and definite view of the object. Upon changing the 

 object-glass, the centering eye-glass should always be 

 employed to ascertain that the centricity of the illumi- 

 nating condenser and microscope body has not been 

 deranged. 



It has been stated that the image of a white cloud oppo- 

 site the sun is the best for illuminating transparent objects 

 when viewed by transmitted light. Mr. Gillett has success- 

 fully imitated this natural surface by an apparatus consist- 

 ing of a large parabolic reflector, with a small camphine 

 lamp on an adjustable stand, having its flame nearly in 

 the focus ; also of two other reflectors of hyperbolic figure, 

 which are employed according to the object-glasses used on 

 the microscope. The parabolic mirror and one of these are 

 attached opposite to each other on the bent arm by which 

 they are supported, having their axes coincident, and ihe 

 enamel disc placed between them. The small hyperbolic 

 reflector receives the light reflected from the large paro- 

 bolic reflector, and concentrates the rays on the small 

 enamel disc. The surface of this disc is roughened, so that 

 the forms of all the incident pencils are broken up, and 

 the effect of a white cloud produced. 



Several important modilications and valuable improve- 

 ments in condensers have been introduced by makers, and, 

 therefore, deserve especial notice. Two things should be 

 required of achromatic condensers intended for general use 

 and for research : First, that the optical combination em- 

 ployed should be adapted to a considerable range of power 

 — say, from an inch, or Jds upwards to the highest ; and, 

 secondly, that they should be capable of working with a 

 large aperture through the ordinary glass slides. When 

 low powers are employed, a pleasantly lit field can be 

 obtained by using a condenser a little out of focus, so that 

 the rays cross before reaching the object ; but a condenser 

 for general use should send a sufficient quantity, and no 

 more, of oblique rays, when in focus, through an object 



