76 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 the 

 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. 



A good mode of imitating artificially the light of a 

 white cloud opposite the sun has been proposed by Mr. 

 Varley : he covers the surface of the mirror under the 

 stage with carbonate of soda, or any similar material, and 

 then concentrates the sun's light upon its surface by a 

 large condensing lens. 



Boss's Achromatic Illuminator, or Condenser. When 

 employing this apparatus, the general practice is to insert 

 in it, as an illuminating lens, the object-glass next lowest 

 in power to that which is intended to be attached to the 

 microscope; so that when the one-eighth is used on the 

 microscope, the one-fourth is screwed into the illuminating 

 apparatus; and so, in like manner, with the rest. But 

 when economy is not regarded, a system of three achro- 



