THE MICROSCOPE. 347 



colored foci is formed ; the violet nearest, and successively, the blue, 

 the green, the yellow, and so on. In whatever place of this series 

 the image may be received, it appears encircled by colored borders, 

 which proceed, according to the distance of the receiving surface 

 from the lens, from the circumstance of the colored rays striking on 

 that surface, either before their union into a focus, or after they have 

 crossed one another therein. Only in the centre of the image of an 

 intercepted white ray is seen the whiteness proceeding from the union 

 and accordance of all the individual colors. All lines and points of a 

 body viewed through a lens, have, on the other hand, a tinted bor- 

 dering, so much the deeper colored and broader, the greater the curva- 

 ture of the lens. 



There are means for obviating in a great measure the effects of 

 spherical aberration : we need only prevent the transmission of rays 

 through the margin of the lens. But the so-called "diaphragm" 

 employed for this purpose, the application of a metallic disk perfor- 

 ated in the middle in front of the lens, increased another inconveni- 

 ence which had been before too sensibly felt in the use of strongly 

 magnifying, and therefore greatly curved and small lenses. The pen- 

 cil of rays passing through such a lens, is itself so small that a very 

 limited space only can be surveyed. And since it can give only a very 

 circumscribed portion of light to the retina, the use of small lenses is 

 thus rendered extremely fatiguing to the eyes. Both defects are in- 

 creased by the application of the diaphragm, setting aside that its 

 employment is imjjossible in the smallest class of lenses. 



The painful and laborious use of single lenses early gave occasion 

 to a different form of the instrument. Here, however, it may not be 

 out of place to say, that common usage gives to magnifying glasses, 

 which, while they enlarge perhaps as much as thirty times, may, 

 when disengaged, be readily managed by the hand, a different name 

 from those which require a fixed frame, in order to keep them immove- 

 able at a determinate distance from the object : the former are styled 

 in German, lupen ; the latter, microscopes. 



A double convex lens depicts, by means of the rays which pass 

 through it from an object in front, an inverted image ; a diminished 

 one when the object is situated at more than its double focal distance, 

 an enlarged one when it is brought nearer, but not so near as the sin- 

 gle distance of the focus, for then no image is any longer formed behind 

 the lens ; all the transmitted rays still diverge. The image is so 

 much the larger, and at the same time further from the lens, the nearer 

 the object is approached to the focus on its anterior side. 



The inverted image formed in this way by a lens is much larger 

 than the apparent extension of the object viewed through the lens when 

 the eye is closely applied to the latter. The former mode of observa- 

 tion also does not impose the condition of so near an approach of the 

 object to the lens. Another related circumstance is, that a larger 

 space can be observed at once than is the case when the lens is brought 

 close before the eye. 



The compound microscope, in its original form, consists of a tube, 

 on the lower end of which is screAved the single lens which first receives 

 the rays proceeding from the object to be observed. By a mechanical 



