10 THE MICROSCOPE. 



explain the difference between large and small pencils of light; 

 those from the small arrow are large pencils; the dotted cones 

 from the large arrow are small pencils. 



In assuming that the whole of this light could have been suf- 

 fered to enter the eye through the lens A B, we did so for the 

 sake of not perplexing the reader with too many considerations 

 at once. He must now learn that so large a pencil of light 

 passing through a single lens would be so distorted by the 

 spherical figure of the lens, and by the chromatic dispersion of 

 the glass, as to produce a very confused and imperfect image. 

 This confusion may be greatly diminished by reducing the 

 pencil; for instance, by applying a stop, as it is called, to the 

 lens, which is neither more nor less than the needle-hole ap- 

 plied to the eye. A small pencil of light may be thus trans- 

 mitted through a single lens without suffering from spherical 

 aberration or chromatic dispersion any amount of distortion 

 which will materially affect the figure of the object; but this 

 quantity of light is insufficient to bear diffusion over the mag- 

 nified picture, which is therefore too obscure to exhibit what 

 we most desire to see those beautiful and delicate markings 

 by which one kind of organic matter is distinguished from 

 another. With a small aperture these markings are not seen at 

 all: with a large aperture and a single lens they exhibit a faint 

 nebulous appearance enveloped in a chromatic mist, a state 

 which is of course utterly valueless to the naturalist, and not 

 even amusing to the amateur. 



It becomes therefore a most important problem to reconcile a 

 large aperture with distinctness, or, as it is called, denfinition; 

 and this has been done in a considerable degree by effecting 

 the required amount of refraction through two or more lenses 

 instead of one, thus reducing the angles of incidence and re- 

 fraction, and producing other effects which will be shortly 

 noticed. This was first accomplished in a satisfactory manner 

 by- 



DR. WOLLASTON'S DOUBLET, 



invented by the celebrated philosopher whose name it bears; 

 it consists of two plano-convex lenses (Fig. 4) having their 

 focal lengths in the proportion of 1 to 3, or nearly so, and 



