MICROSCOPE. 



353 



When condensed light is employed with 

 deep powers, great care is necessary in order 

 to bring out the best effects of a microscope, 

 with difficult objects. These remarks apply to 

 the simple as to the compound forms of the 

 instrument. We do not ourselves consider the 

 ordinary condenser as of much more value for 

 the higher than for the lower class of trans- 

 parent objects ; and we think that it may be 

 discarded from the instrument without dis- 

 advantage. Great assistance may be obtained, 

 however, from a well-constructed condenser, 

 in the resolution of the more difficult class of 

 microscopic objects. That which was pro- 

 posed by Dr. Wollaston for use with his 

 doublet consists of a tube about six inches 

 long, having at the lower end a diaphragm 

 with a circular perforation about three-tenths 

 of an inch in diameter, through which light, 

 proceeding from a radial point or surface, is 

 reflected by a mirror below it. At the upper 

 end of the tube is a plano-convex lens, about 

 three-fourths of an inch focus, with its plane 

 side next the observer; the object of which is 

 to form a distinct image of the circular per- 

 foration in the plane of the object, which 

 should be at about eight-tenths of an inch 

 from the lens. We consider this instrument 

 to be theoretically faulty; inasmuch as the 

 point from which the illuminating rays di- 

 verge, and the limiting aperture are not co- 

 incident, so that pencils brought to a focus for 

 the former are not for the latter. The length 

 of the tube, again, is an inconvenience, espe- 

 cially in the case of small microscopes. Never- 

 theless, it is much superior to the ordinary 

 form of condenser. An improvement was 

 suggested by Dr. Goring, which consisted in 

 shortening the tube below the lens, and re- 

 moving the stop from that end of it to the 

 other, so that it should be just beneath the 

 object; in this manner the illuminating rays 

 may be brought to a focus on the object, the 

 superfluous ones being cut off by the stop. 

 We have derived much advantage from the 

 use of this condenser in the small doublet mi- 

 croscope already noticed, and by a slight mo- 

 dification of it we can obtain a variety of 

 illumination, which is often very useful. Our 

 condenser consists of three tubes, one sliding 

 within the other; the outer one is fixed to the 

 under side of the stage; the second carries at 

 its upper end the stop, the distance of which 

 from the object may thus be changed, and the 

 inner one carries the condensing lens. A stop 

 may be screwed into the bottom of the latter, 

 if it is desired to adopt Dr. Wollaston's plan. 

 We have derived the greatest advantage from 

 the use of this condenser, however, by using 

 it with direct light from a radiant point at 

 some distance, a bright cloud, for instance, 

 in the day-time, and a lamp or candle on the 

 opposite end of the table at night. In either 

 of these cases, the focus of the illuminating- 

 rays may be made coincident with the plane 

 of the object, without that glare which will 

 almost certainly be produced if the source of 

 light is nearer and more intense. When thus 

 used, Dr. Goring's condenser approaches in its 



VOL. III. 



character to that of Sir D. Brewster,* which 

 we shall now describe, adding some ideas of 

 our own in reference to its construction. 



The principle of illumination on which Sir 

 D. Brewster lays great, and we think fully- 

 deserved stress, is, that the focus of the illu- 

 minating rays shall be coincident witli the 

 object, so that there shall not be two sets of 

 rays at different angles, one proceeding from 

 the luminous object and the other from the 

 object to be magnified. This can only be 

 attained in any degree by making the image 

 of the illuminating body coincident with the 

 object; and it will be perfectly accomplished, 

 in proportion as the rays forming that image 

 are themselves free from aberration. If, for 

 example, the white rays have been separated 

 into their component colours by the condenser, 

 these colours will be imparted to the object, 

 the appearance of which will also be rendered 

 less distinct by the spherical aberration of the 

 condensing lens. Hence an achromatic lens, 

 or, if this be objected to on account of its 

 expense, a Herschel's aplanatic doublet, should 

 be used as the condenser ; the latter we have 

 found to answer very well, as the centre of 

 the circle illuminated by it is very nearly free 

 from false colour. Further, if a mirror be 

 employed to change the course of the rays, 

 it should be of metal, in order to avoid the 

 false rays reflected from the first surface of the 

 glass. If day-light be employed, no other 

 precaution is necessary; but if the illumination 

 be obtained from a lamp or candle, it will be 

 necessary to limit the amount of light ad- 

 mitted. This must not be by a stop placed 

 beneath the lens, for the reason already speci- 

 fied ; but it should be accomplished by a stop 

 or shade placed as near as possible to the flame, 

 so that the image of that and of the flame 

 may be brought virtually if not exactly to 

 the same focus. We have found that the same 

 end may be attained by removing the lamp or 

 candle to a greater distance, so as to diminish 

 the intensity of the light to the required 

 amount, and only a very low flame will then 

 be required, as the condensation of the rays 

 is much more perfect than with the ordinary 

 lens. The achromatic condenser is strongly 

 recommended also by M. Dujardin, an eminent 

 microscopist of France ; and we know that it 

 is now considered an indispensable addition to 

 microscropes of the highest class, affording, 

 as it does, the means of resolving objects, 

 previously considered too difficult to admit of 

 a clear view of their nature. We are disposed 

 to think, however, that some improvement is 

 necessary in order to develope the highest 

 powers of this instrument. The rays of light 

 proceeding from the radiant point or object, 

 being brought to a focus by the condenser on 

 its surface, cross each other there, and should 

 proceed to the object-glass of the microscope, 

 as if they came from the object itself. Now, 

 unless they are made to converge upon the 

 object at the same angle at which they diverge 



* Treatise on the Microscope, from the Ency- 

 clopaedia Britannica. 



2 A 



