CO THE MICROSCOPE. 



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 illuminating condenser 

 and microscope body has not been deranged. 



It has been stated that the image of a white cloud opposite the sun 

 is the best for illuminating transparent objects when viewed by trans- 

 mitted light. Mr. Gillett has successfully imitated this natural surface 

 by an apparatus consisting 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 em- 

 ployed 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 coin- 

 cident, and the enamel disc placed between them. The small hyper- 

 bolic reflector receives the light reflected from the large parobolic re- 

 flector, and concentrates the rays on the small enamel disc. The sur- 

 face 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. 



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 achromatic com- 

 binations is supplied, adapted for the illumination of the whole range 

 of the powers of the microscope : the whole system being employed for 

 the highest powers ; two of such combinations with the middle powers ; 

 and the largest combination by itself for the lowest powers. This illu- 

 mination is not required for objects when viewed with object-glasses 

 transmitting small pencils of rays, or whose angular aperture is less 

 than thirty degrees; that is, where the object-glass is of greater focal 

 length than half an inch. 



The apparatus is fixed to the under side of the stage of the micro- 

 scope, in the place of the diaphragm-plate; and before fixing, the proper 

 object-glass, as an illuminating lens, must be screwed on to it. In 

 fig. 40, two tubes are seen sliding one within the other ; to the outer 

 one, 6, is attached a flat plate a, which slides underneath the stage, 

 and is adjusted for distance by the screwy,* at c the milled-head is 



