THE MICROSCOPE. 



19 



aperture at G, which is supposed to be nearly the place of the 

 object. The object is sometimes best seen when the image of 

 the aperture is also best seen; and sometimes it is best to ele- 

 vate the summit G of the cone A B G above the object, and at 

 others to depress it below : all which is done at pleasure by the 

 power of moving the lens A B. If artifical light (as a lamp or 

 candle) be employed, the flame must be placed in the principal 

 focus of a large detached lens on a 

 stand, so that the rays L L may fall in 

 parallel lines on the mirror, or as they 

 would fall from the cloud. This will be 

 found an advantage, not only when the 

 Wollaston condenser is employed, but 

 also when the mirror and diaphragm are 

 used. A good mode of imitating arti- 

 ficially the light of a white cloud op- 

 posite the sun has been proposed by 

 Mr. Yarley; he covers the surface of the 

 mirror under the stage with carbon- 

 ate of soda or any similar material, 

 and then concentrates the sun's light 

 upon its surface by a large condensing 

 lens. The intense whie light diffused 

 from the surface of the soda forms an 

 excellent substitute for the white cloud, 

 which, when opposite the sun, and of 

 considerable size, is the best daylight, as 

 the pure sky opposite to the sun is the 

 worst. 



The Compound Microscope may, as be- 

 fore stated, consist of only two lenses, 



while a simple microscope has been shown to contain sometimes 

 three. In the triplet for the simple microscope, however, it 

 was explained that the effect of the two first lenses was to do 

 what might have been accomplished, though not so well, by 

 one; and the third merely effected certain modifications in the 

 light before it entered the eye. But in the compound micro- 

 scope the two lenses have totally different functions; the first 

 receives the rays from the object, and, bringing them to new 



Fig. 11. 



