MICROSCOPE 



181 



lenses the errors are still further reduced by the 

 close approximation of the lenses to the object, 

 which causes the refraction to take place near the 

 axis, and thus we have a still larger pencil of light 

 transmitted, and have also a more distinct and 

 vivid image presented to the eye. 



Simple Microscope. By this term we mean an 

 instrument by means of which we view the object 

 through the lens directly. These instruments may 

 be divided into two classes those simply used in 

 the hand, and those provided with a stand or frame, 

 so arranged as to be capable of being adjusted by 

 means of a screw to the exact focal distance, and 

 of being moved over different parts of the object. 

 The single lens used may lie either a bi-convex or 

 a plano-convex. When a higher power is wanted 

 a doublet, such as we have already described, may 

 be employed, or a Coddington lens, 

 whioli consists (fig. 4) of a sphere in 

 which a groove is cut and failed up 

 with opaque matter. This is perhaps 

 the most convenient hand lens, as it 

 matters little, from its spherical form, 



Fi- 4. *" w ' lnt jx^i'ion it is held. In the 

 simple microscope single or combined 

 lenses may be employed, varying from a quarter to 

 two inches. There are many different kinds of 

 Dtands for simple microscopes made, but, as they 

 are principally used for dissection, the most import- 

 ant point next to good glasses is to secure a firm, 

 large stage for supporting the objects under exam- 

 ination. When low powers alone are used the 

 stage-movements may be dispensed with ; but 

 when the doublet or triplet is employed some 

 more delicate adjustment than that of the hand is 

 necessary. 



Compmtnd Microscope. In the compound micro- 

 scope in its simplest form the olwerver does not 

 view the object directly, but an inverted real image 

 or picture of the object is formed by one lens or set 

 of lenses, and that image is looked at through 

 another lens. The compound microscope consists 

 of two len-es, an oliject and an eye lens ; but each 

 of these may be compounded of several lenses play- 

 ing the part of one, as in the simple microscope. 

 The eye-lens, or ocular, is that placed next the 

 eve, and the object lens, or objective, that next 

 the object. The objective is generally made of 

 two or three achromatic lenses, \\ Idle the eye-piece 

 generally consists of two plano-convex lenses, with 

 their flat faces next the eye, and separated at half 

 the sum of their focal lengths, with a diaphragm 

 or stop l>etween them. Lenses of high power are 

 so small as to admit onlv a very small beam of 

 light, and consequently what is gained in magnify- 

 ing power is often worthless from deficient illumina- 

 tion. Various devices have lieen employed to over- 

 come this difficulty. The light may lie concentrated 

 by achromatic condensers placed lieneath the stage, 

 or the curvature of the lens may be such as to allow 

 as large a number of divergent rays as possible to 

 impinge upon it. Such a lens is said to have a 

 large 'angle of aperture,' the angle of aperture 

 lieing that inado by two lines converging from the 

 margins of the lens to its focal point. Recently 

 lenses, termed 'immersion lenses, have been con- 

 structed, of such a curvature that when immersed 

 in a drop of liquid placed over the object light is 

 admitted on all sides. With an immersion lens 

 there i- high magnifying power with sufficient 

 illumination. 



The accompanying diagram (fig. 5) explains the 

 manner in which tiie complete compound micro- 

 seope aets. \\'e have here represented the triple 

 achromatic objective, consisting of three achro- 

 matic lenses combined in one tube, in connection 

 with the eye-piece, which now consists of the lield- 

 glass, FF, in addition to the eye-glass, . The 



function of the field-glass, FF, is that the rays of 

 light from the object tend, after traversing the objec- 

 tive, to form an image at AA ; but coming in contact 

 with the lield-glass, FF, they are 

 bent, ami made to converge at BB, 

 where a real image is formed, at 

 which place a stop or diaphragm is 

 placed to intercept all light, except 

 what is required to form a distinct A , 

 image. From BB the rays proceed \ 

 to the eye-glass, EE, exactly as they If II 

 do in the simple microscope. The * B ' '"- 

 real image formed at BB is there- 

 fore viewed as an original object 

 through the eye-glass, EE. The 

 lens, FF, is not essential to a com- 

 pound microscope ; but as it is 

 quite evident that the rays pro- 

 ceeding to AA would fall exterior 

 to the eye-lens, EE, if it were 

 removed, and only a part of the 

 object would thus be brought under 

 view, it is always made use of in 

 the compound microscope. 



A mirror is placed under the stage 

 for retlecting the light through the 

 object under observation. This 

 method of illumination by trans- 

 mitted light is used when the 

 object is transparent. When 

 opaque, light i.s reflected on the 

 oliject by a bull's-eye lens, called 

 a condenser. The best instru- 

 ments are supplied with six or 

 seven object-glasses, varying in 



Fig. 5. 



magnifying power from 20 to 2500 

 diameters. The eye-pieces supplied 

 are three in number, each of which 

 consists of two plano-convex lenses, between which 

 a stop or diaphragm is placed, half-way between 

 the two lenses. As the magnifying power of a 

 compound microscope depends on the product of 

 the magnifying powers of the object-glass and the 

 eye-piece, it follows that its power may be in- 

 creased or diminished by a change in either or 

 lioth of these 

 glasses. In the 

 mechanical ar- 

 rangements it 

 is of import- 

 ance to nave 

 the instrument 

 so constructed 

 that, while 

 every facility 

 is afforded for 

 observation 

 and easy ad- 

 justment, there 

 should also Im 

 great steadi- 

 ness. These 

 ends are 

 achieved in 

 various ways, 

 of which fig. 6 

 is one of the 

 simplest: a, 

 brass stand, 

 supported on Fig. 6. 



three feet ; b, 



mirror supported on trunnions ; c, diaphragm, 

 pierced with circular holes of various sizes, to regu- 

 late the admission to the object of reflected light 

 from the mirror ; d, stage-plate, on which the object 

 is placed; e, screw, with milled head for fine ad- 

 justment ; f, the object-glass or objective ; g, brass 

 tube in which the body of the instrument is moved, 



