354 



MICROSCOPE. 



to enter the objective, we cannot but tbink that 

 a source of error still remains, and that the 

 most perfect image possible, formed by an 

 achromatic object-glass, of an object which is 

 artificially illuminated, can only be produced 

 when the rays from the source of light take 

 exactly the same course as if they proceeded 

 from the object itself. That they may have 

 this course, they must be made to converge 

 upon the object by a condensing lens, whose 

 focus for parallel rays (if the illuminating point 

 be very distant) shall be the same as the acting 

 focus of the objective. A different condenser 

 would thus be required for every objective ; 

 but the expense of these would be sufficient 

 to preclude their general employment. The 

 same condenser might be employed, however, 

 for several low powers; the highest having 

 their own expressly adapted to them. We 

 may mention it as a fact for which we cannot 

 very well account, that we have been able to 

 obtain a very beautiful and distinct illumina- 

 tion by the use of an aplanatic- doublet con- 

 denser, receiving its rays from the ground glass 

 globe of the common table-lamp, which would 

 not seem to furnish any of the conditions that 

 we have dwelt on as of theoretical impor- 

 tance. 



Opaque objects may be illuminated in two 

 principal ways, either by a light cast obliquely 

 upon them by a condensing lens, or by rays 

 thrown upon them perpendicularly by a silver 

 speculum fixed to the object-end of the body, 

 which receives them from the mirror below. 

 Both these modes have their peculiar advan- 

 tages, and will be found by experience to be 

 applicable with advantage to different classes 

 of objects. No general directions on the sub- 

 ject can, however, be given. The condenser 

 to be used by lamp-light for large opaque ob- 

 jects should be a bull's-eye or hemispherical 

 lens of four inches in diameter ; by this, 

 even from a common candle or flat-wicked 

 lamp, a sufficient light may be attained for 

 almost any purpose, and with an Argand lamp 

 a very powerful illumination is obtained. For 

 the parallel rays of day-light, however, an 

 ordinary double-convex lens of the focus of 

 two inches or more may be employed; this 

 may be mounted upon a separate stem and 

 foot, as the bull's-eye should always be, or it 

 may be attached to some part of the micro- 

 scope itself. We have seen foreign instru- 

 ments in which it was fixed to the object-end 

 or nose of the body ; a construction which we 

 deem essentially bad, inasmuch as it then re- 

 quires to be readjusted every time that the 

 focal distance is changed by the substitution of 

 one objective for another. A better mode in 

 our opinion is to attach it to the side of the 

 stage by a jointed arm possessing a pin which 

 may be fitted into one of three or more holes 

 drilled in the side of the stage with sufficient 

 tightness to remain in any position. If the 

 wire to which the lens-frame is attached be 

 made to pass through a sprung socket con- 

 nected by a ball-and-socket-joint with the 

 jointed arm, an immense variety in position 

 may be readily given to the condensing lens, 



which will render a separate mounting for it 

 unnecessary. 



The metallic speculum, or Lieberkuhn, for 

 throwing down rays directly upon the object, 

 is sometimes attached to the objective itself, 

 sometimes to a tube which maybe drawn down 

 over it to the required amount. The former con- 

 struction is the most perfect, each magnifier 

 having its own speculum; but the latter is 

 most economical, as one speculum serves for 

 many objectives. When the latter construc- 

 tion is employed, the tube should be marked 

 with the numbers of the magnifiers at the 

 points at which it may be best adjusted for 

 each, so that, when the object is in the focus 

 of the objective, it may be in full receipt of 

 the rays reflected from the speculum. In this 

 mode of illumination the size of the concave 

 mirror will be found to have a considerable 

 influence ; and the capability of adjusting also 

 its distance from the speculum gives a useful 

 variety to the mode of illumination. It can 

 never be too fully kept in mind, that, in the 

 examination of doubtful objects, no really satis- 

 factory result can be attained, until they have 

 been viewed in every possible way. When the 

 object is not perfectly opaque, or does not fill 

 up the whole of the field of view, it should 

 always be placed on an opaque disc, which 

 should be large enough to interrupt all the rays 

 passing directly upwards from the mirror to the 

 eye. The colour of this disc may be advan- 

 tageously varied for certain objects, but in 

 general we consider a dead black the best for 

 giving them effect. As a general rule it may 

 be remarked, that the fewer the rays entering 

 the eye except through the object, the more 

 perfect will be the view of it ; and if there be 

 anything approaching to a glare around an 

 opaque object, whether the light proceed di- 

 rectly from the mirror, or be reflected back 

 from the too-bright surface of the disc, it is 

 equally injurious, and will occasion a mis- 

 tiness over the object itself. It is evident, how- 

 ever, that the size of the disc must be governed 

 by that of the field of the objective employed ; 

 for, if too large, it will interrupt too much of 

 the light impinging on the speculum. A piece 

 of black glass, mounted upon a long strip of 

 common window-glass, will be found very 

 useful where the object is such that it can 

 be laid upon it when the microscope is in- 

 clined. When the object requires to be held 

 in the stage-forceps, however, a disc of black 

 card may be placed behind it, so as to afford 

 it a back-ground. And for some delicate 

 opaque objects, it is most advantageous to em- 

 ploy little concave discs or cups, with their 

 interior blackened, in front of which the ob- 

 jects are to be placed. Similar dark back- 

 grounds are useful when oblique light is em- 

 ployed. 



III. MAGNIFYING POWER OF MICRO- 

 SCOPES. 



The next point to which we shall advert is 

 the mode of estimating the magnifying power 

 of microscopes, and of measuring the real 

 size of objects under examination. Our esti- 



