MICROSCOPE. 



339 



magnifying power is employed; and the strain- 

 ing of the eye, which is occasioned by their 

 very minute aperture. Thus a triplet in our 

 possession, which will show the most difficult 

 test-objects, has a focal distance of only about 

 3 ' 3 th of an inch, and an aperture through which 

 the smallest pin would scarcely pass. But the 

 first of these disadvantages is more apparent 

 than real. The object should be always co- 

 vered with talc, (which may be easily split into 

 laminse of the g^th of an inch in thickness,) for 

 the purpose of protecting both it and the lens 

 from injury by accidental contact ; and the 

 magnifier should not be screwed into the arm 

 which carries it, but loosely fitted, so that, if the 

 observer should happen to bring the arm too 

 near the stage, he may not force down his lens 

 upon the object. As Mr. Holland justly ob- 

 serves, " Should the proximity of the object 

 to the lowest lens of the triplet be urged as a 

 material objection to its usefulness, it may be 

 answered that the whole microscope is a mass 

 of delicacies ; consequently, it cannot be al- 

 lowed that a line be arbitrarily drawn, beyond 

 which every thing is to be considered as too 

 delicate." The second of the above objections 

 must be obviated by never continuing the use 

 of deep powers in a simple microscope for any 

 length of time at one sitting, and by taking 

 care to adjust the instrument in such a manner 

 that the head may be as little inclined forwards 

 as possible. 



The only other form of simple microscope 

 which we shall notice is one commonly known 

 under the name of the Coddington lens. The 

 first idea of it was given by Dr. Wollaston, 

 who proposed to cement to- ether two plano- 

 convex, or hemispherical lenses, by their plane 

 sides, with a stop interposed, the central aper- 

 ture of which should be equal to ith of the 

 focal length. The great advantage of such a 

 lens is that the oblique pencils pass, like the 

 central ones, at right angles with the surface ; 

 and that they are consequently but little subject 

 to aberration. The idea was further improved 

 upon by Mr. Coddington, who pointed out 

 that the same end would be much better an- 

 swered by taking a sphere of glass, and grind- 

 ing away the equatorial parts, the groove being 

 then filled with opaque matter, so as to limit 

 the central aperture. Such a lens gives a large 

 sphere of view, admits a considerable amount 

 of light, and is equally good in all directions; 

 but its powers of definition are by no means 

 equal to those of an achromatic lens, or even 

 of a doublet. This form is very useful, there- 

 fore, as a hand lens, in which a high power is 

 not required, but has no particular advantages 

 for magnifiers of short focus, nor for the object- 

 glasses of a compound microscope.* 



It may be desirable to mention that a magni- 

 fier, now known under the name of the Stan- 

 hope lens, and much praised by those interested 



* We think it right to state that many of the 

 magnifiers sold as Coddington lenses are not really 

 (as we have satisfied ourselves) portions of spheres, 

 but are manufactured out of ordinary double-con- 

 vex lenses, and will be destitute, therefore, of 

 many of the above advantages. 



Fig. 162. 



in its sale, is nothing more than a double coa- 

 vex glass, much thicker than ordinary, so that 

 an object in contact with one of its surfaces 

 shall be in focus to the eye placed behind the 

 other. This is an easy method of applying 

 rather a high magnifying power to scales of 

 butterflies' wings and other similar flat and 

 minute objects; but the instrument is totally 

 destitute of value as a means of scientific re- 

 search, and must be regarded as an ingenious 

 philosophical toy. 



Compound Micro- 

 scope. - - The com- 

 pound microscope es- 

 sentially consists, as 

 already stated, of two 

 lenses, which are so 

 disposed that one of 

 them receives the rays r, 

 of light from the object, 

 and forms an image by 

 its refraction of them ; 

 and this image is seen 

 by the eye through the 

 second lens, which acts 

 upon it as a simple mi- 

 croscope. The princi- 

 ple of such a micro- 

 scope will be at once 

 understood from the 

 adjoining diagram (fig. 

 162). According to the 

 laws already stated, if 

 the object be at a less 

 distance from the lens 

 than its diameter of cur- 

 vature (supposing it to 

 be a double - convex 

 lens, or twice that dis- 

 tance of a plano-con- 

 vex) the image will be 

 larger than the object ; 

 and this in proportion 

 as the latter is brought 

 nearer to the principal 

 focus, at which it can 

 give rise to no image, 

 as its rays after refrac- 

 tion become parallel. 

 Hence, by the use of 

 the same object-glass, 

 a considerable variety 

 of power might be ob- 

 tained ; for, if the image 

 be formed near the lens, 



it will be small; but if . 



., , . . i j , A B, the object, of which 



the object be caused to an ' ampli j fied ' is 

 approach it, the image f orme d by the object- 



I 



will be thrown toacon- 

 siderable distance, and 

 will be proportionably 

 magnified. The eye- 

 piece would of course 

 require, however, a cor- 

 responding re-adjust- 

 ment; and, in fact, 

 the construction of 

 the whole instrument 

 would need modifica- 



glass C D, in the con- 

 trary direct ion, at A' 15', 

 by the convergence of 

 the rays of the several 

 pencils to a focus. These 

 again diverging are re- 

 ceived by the eye-glass 

 L M, which gives them 

 the nearly parallel di- 

 rection necessary for 

 them to enter the eye, 

 and causes the apparent 

 sizeoftiie image to bj E. 

 z 2 



