MICROSCOPE. 



mate of the magnifying power of a microscope 

 must depend upon the standard which we 

 assume as the ordinary distance at which the 

 object is seen with the naked eye; since, if 

 brought within five inches, it is seen under 

 double the angle, and therefore of double the 

 size which it appears to possess at ten inches. 

 If, therefore, the former distance were taken as 

 a standard, the magnifying power of a lens or 

 microscope will only be half that at which it 

 would be estimated with reference to the other. 

 Nearly all opticians, however, have agreed in 

 considering ten inches as the standard of com- 

 parison ; and when, therefore, an object is 

 figured as magnified 100 diameters, it is meant 

 that this figure placed at ten inches distance 

 from the eye is 100 times the dimension, each 

 way, of the real object seen with the eye alone 

 at a similar distance. The measurement of 

 the magnifying power of simple or compound 

 microscopes by this standard is attended with 

 no difficulty. All that is requisite is to have 

 a glass or thin slip of ivory accurately divided 

 to a small fraction of an inch d^jth will usually 

 answer very well), and a common foot-rule, 

 divided to tenths of an inch. The glass or 

 ivory micrometer being adjusted to the focus 

 of the magnifier, the rule is held parallel with 

 it, at the distance of ten inches from the eye. 

 If the second eye be then opened, whilst the 

 other is looking at the object, the circle of light 

 included within the field of view, and the 

 object itself will be seen faintly projected upon 

 the rule ; and it will be very easy to mark upon 

 the latter the apparent distances of the divi- 

 sions on the micrometer, and thence to ascer- 

 tain the magnifying power. Thus, supposing 

 that each of the divisions of T L,th of an inch 

 corresponded with 1J inch upon the rule, the 

 linear magnifying power is 150 diameters; 

 if it corresponded with half an inch, the mag- 

 nifying power would be 50 diameters. The 

 superficial magnifying power is of course esti- 

 mated by squaring the linear ; but this is a 

 mode of statement never adopted by scientific 

 observers, although often employed to excite 

 popular admiration or to attract customers by 

 those whose interest is concerned in doing so. 



When the magnifying powers of the several 

 objectives of a microscope are known, it is easy 

 to make a fair approximation to the real size of 

 an object under examination, by projecting its 

 image, as before, upon a foot-scale held at ten 

 inches distance ; and the apparent dimensions 

 it there exhibits, divided by the magnifying 

 power employed, will of course be its real size. 

 More accurate measurements are generally re- 

 quired, however, although the foregoing may 

 serve as a sufficiently near approximation for 

 ordinary purposes. Various methods of ob- 

 taining these have been devised. The most 

 perfect measurements are obtained by a fine 

 micrometer-screw, by turning which either the 

 object or the body will be made to execute a 

 transverse movement ; just as, in the fine ad- 

 justment of the focus, the body is made to 

 approach or recede from the stage. This ap- 

 paratus may be attached to the stage, if it be 



thought preferable to move the object, or to 

 the arm if the body be made to traverse. In 

 proportion to the fineness of the screw, and to 

 the size of the milled-head, will measurements 

 be obtained of minute accuracy. In the focus 

 of the eye-piece a very fine thread is placed ; 

 and one edge of the image being brought 

 against it, the position of the micrometer is 

 noticed ; and the other edge being then brought 

 to the same line, the number of divisions of 

 the micrometer-screw, which have passed over 

 the index, will indicate its size. The expen- 

 siveness of this micrometer, when made with 

 sufficient accuracy and minuteness, is the only 

 bar to its general employment. Other means 

 have been devised, however, which are scarcely 

 inferior; and the simplest of these requires 

 only the use of an eye-piece of a construction 

 different from the ordinary, with coarsely di- 

 vided glasses. The micrometer eye-piece is 

 made upon the principle of Ramsden's. The 

 optical part of it differs from that in common 

 use, in having the plane side of the field-glass 

 turned towards the object; and in the adjust- 

 ment of the foci of its lenses in such a manner 

 that the image to be viewed by it must be 

 beneath the field-glass instead of being be- 

 tween it and the eye-glass. In its focus there 

 is placed a plane glass with divisions on it ; 

 these may be from J th to T ^th of an inch 

 apart ; the latter will enable very minute mea- 

 surements to be taken. By this arrangement, 

 when the object is brought into focus, it ap- 

 pears as if it were traversed by cross lines; 

 since its image is coincident with the divided 

 glass, and is, like it, viewed by the eye-piece 

 as by a simple microscope. The value of these 

 divisions will, of course, depend upon the 

 degree in which the object is magnified in the 

 image ; and they must be ascertained for every 

 objective, by the divided glass or ivory mi- 

 crometer. This being brought into focus, and 

 so placed that the direction of its two sets of 

 lines shall correspond with those of the divided 

 glass in the eye-piece, the number of divisions 

 in the latter, corresponding to each division of 

 the former, must be observed, and their value 

 will be thus ascertained. Thus, if one divi- 

 sion on the ^jjth inch micrometer be found to 

 coincide with eight on the eye-piece, these 

 eight will together indicate a dimension of 1 J 5 tli 

 of an inch upon the object; and each division 

 of the eye-piece will of course be equivalent 

 to the gjjjth of an inch. If there is not an 

 exact correspondence, or if it be desired to 

 obtain a power which can be expressed in 

 round numbers, a little alteration in the length 

 of the body, made by drawing out his eye- 

 piece, will enable the microscopist to effect 

 this ; but the eye-piece should be marked, so 

 as to be adjustible to the same point again, 

 when the same magnifying power is employed. 

 When the value of the divisions on the glass 

 in the eye-piece is ascertained for every mag- 

 nifier, the object-glass micrometer may be put 

 aside altogether ; since by the use of the eye- 

 piece alone, a series of lines, the real distance 

 corresponding to which is known, is projected 



2 A 2 



