346 THE MICROSCOPE. 



radius extremely small, and consequently of enormous magnifying 

 power. We may obtain very small and almost exactly spherical lenses 

 by drawing out a glass thread and melting off small drops from it. 

 Globules thus obtained, and set in a fitting manner, afford a linear 

 enlargement of as much as 2,000, (or if we accept the authority of the 

 itinerant microscope-exhibitors, to whom we shall return hereafter, an 

 enlargement of just 8,000 million times;) but this rate of increase 

 has never been used or pretended to be used in scientific researches. 

 In practice, to be sure, no limit as regards the magnifying power of 

 single lenses is to be too strictly regarded: but a train of radical defects 

 makes the profitable employment of such results as those just alluded 

 to impossible. 



The rays which impinge on the center of a lens and the points im- 

 mediately around it are more slightly refracted than those which enter 

 nearer the edge. The points at which the rays proceeding from an 

 object are united do not coincide, but present a series of images lying 

 closely behind one another. The image of the sun, received with a 

 single lens, appears at the distance from the lens where the image is 

 brightest and clearest (the focus of the rays falling in and near the 

 center) surrounded by a visible fringe, the cause of which is, that the 

 rays entering towards the edges of the lens, after crossing one another 

 at their respective points of union, situated in front of the receiving 

 surface, fall upon that surface in the circumference of the principal or 

 brightest image. The name of spherical aberration has been given to 

 this unequal refraction of rays transmitted through a lens. In like 

 manner the rays passing through the lens will not be collected by the 

 e^ye into a sharply-defined image. Those entering at the edge, fringe 

 the image of those which have passed through the middle. The image 

 of a small object is injuriously circumscribed, that of a large one dis- 

 torted; inconveniences, which, with the progressively increased power 

 of the lens, become at last intolerable. 



The second inconvenience, increasing with the augmented power of 

 single lenses, is, that objects seen through them appear surrounded by 

 colored borders. A ray of white light, in consequence of being bent 

 in its passage from one transparent body to another, is not symmetri- 

 cally refracted, but separated into rays of different colors, whose refrangi- 

 bility is unequal. Of the variously colored rays, into which white light 

 thereby is resolved, the violet is most, the red least diverted from its 

 original course. From the transmission of white light through a 

 prism arises the well known seven-colored spectrum, whose red rays 

 lie nearest, the violet farthest from the point, at which the prolonga- 

 tion of the original direction of the beam of white light would strike 

 the surface on which the spectrum is formed. 



By transmission through a lens the violet rays, in virtue of their 

 greater refrangibility, will be collected into a focus nearer the lens, 

 the red ones further off. As in consequence of the spherical aberra- 

 tion of light a succession of foci exists at different distances from 

 the lens, the rays from the outer portions of the lens uniting nearer 

 to it, those from the middle further off, so, through the separation of 

 the white light into differently colored rays, a series of differently 



