

MICROSCOPE. 



(131 



sJA 



ad UMO concentrates the sun's light upon iU surface by a luge onn- 

 d<Minc lens. The intaaM whit* light diffused from the surface of the 

 wxU forms u> excellent substitute fur the whit* cloud, which, when 

 oppuwte the tun Mid of oooddetmbU aia, it the beat day-light, as the 

 pun sky opposite to the mm u the wont. 



Tkt Oomf**d ilirntropt my. a* before stated, consist of only two 

 lra**s, while simple microscope hu been shown to contain sometime* 

 three. In the triplet for the simple microscope, however, it WM 

 explained that the eflect of the two Brat lenen WM to do what mu-ht 

 here bean accomplished, though not so well, by one ; and tl 



" r effected certain modifications in the light before it entered the 

 ut in the compound microscope the two lenaee have totally 

 function* ; the first receive* the rayi from the object, and, 

 bringing them to now foci, form* an image, which the second Inn 

 trenU as an original object, and magnifies it jurt as the single micro- 

 Mope magnified the object itself. 



The annexed figure (12) shows the course of the rays through acorn- 

 ,. pound microscope of two lenses. The rays' 



proceeding from the object A B are so acted 

 upon by the lens c n. near it, and thence 

 called the objective, that they are converged 

 to foci in A'B', where they form an enlarged 

 image of the object, aa would be evident if 

 a piece of oiled paper or ground glass were 

 placed there to receive them. They are not 

 so intercepted, and therefore the image is 

 not rendered visible at that place ; but their 

 further progress is similar to what it would 

 have been had they really proceeded from 

 an object at A'B'. They are at length re- 

 ceived by the eye-lens L u, which acts upon 

 them as the simple microscope has been 

 described to act on the light proceeding 

 from its objects. They are bent so that 

 they may enter the eye at F. in parallel 

 lines, or as nearly so as U requisite for 

 distinct vision. When we say that the rays 

 enter the eye in nearly parallel lines, we 

 mean only those which proceed from one 

 point of the original object. Thus the two 

 parallel rays 11 E have proceeded from and 

 are part of the cone of rays CAD, emanating 

 from the point A of the arrow ; but they do 

 not form two pictures in the eye, because 

 any number of parallel rays which the pupil 

 can receive will be converged to a point by 

 the eye, and will convey the impression of 

 one point to the mind. In like manner the 

 rays L E are part of the cone of rays emanating 

 from B, and the angle I, E M is that under 

 which thu eye will see the magnified image 

 of the arrow, which is evidently many times 

 greater than the arrow could be made to 

 occupy in the naked eye at any distance 

 within the limits of distinct vision. The 

 magnifying power depends on two circum- 

 stances : Hint, on the ratio between the 

 anterior distance A c or n t> and the posterior 

 focal length c u or n A'; and secondly, on the 

 power of the eye-Ions L H. The first ratio in 

 the same as that between the object A B and 

 the image A'B' ; this and the focal length or 

 power of the eye-lens are both easily obtained, 

 and their product is the power of the com- 

 pound instrument. 



Since the power depends on the ratio 

 between the anterior and posterior foci of 

 the objective, it is evident that by increasing 

 that ratio any power may be obtained, the same eye glass being used ; or, 

 having determined the first, any further power maybe obtain. <l by 

 increasing that of the eye-glass ; and thus, by a pre-arrangemcnt . i tl, 

 relative proportion* in which the magnifying power shall be divided 

 between the objective and the eye-glass, almost any given distance 

 (within certain limits) between the first and its object may be > 

 This is one valuable peculiarity of the compound instrument ; and 

 another U the large field, or large angle of view, which may be obtained, 

 very pert of which will be nearly equally good ; whereas with the best 

 simple microscopes the Held is small, and is good only in the centre. 

 The field of the compound instrument is fn -od by using 



two glasses at the eye-end ; the first being called, from its purpose, 

 the brld gUi. and the two constituting what U called the eye-piece. 

 Tlii.. will be more particularly explained in the figure of the achromatic 

 compound micracope presently given. 



For upwards of a century the compound microscope, notwithstanding 

 the advantages .-, | WM comparatively feel. 



nMrunMnt, ow.ng iot| 10 di<Unoe which the light had to 

 trovers*, and the con- *. of the chromatic and spherical 



aberration*. To ei|.lain this we have drawn in fg. 1 2 a second image 



near A'B', the fact being that the object-glass would not form one 

 image, as has been supposed, but an infinite number of variously- 

 oolouml and various-sued image*, occupying the space between the 

 two dotted arrows. Those nearest the object-glass would be red, and 

 those nearest the eye-glass would be blue. The effect of this is to 

 produce so much confusion, that the instrument was reduced to a 

 mere toy, although these errors were diminished to the utmost possible 

 exU-nt by limiting the aperture of the object-glass, and thus resti 

 the angle of the )H.-ncil of light from each point of the object, 

 this involved the defects, already explained, of making the picture 

 obscure, so that on the whole the IK-HI compound : I were 



inf. rior to the simple microscopes o{ a single lens, with which indeed 

 all the important observations of the last century were made. 



Kven after tin- improvement of the simple microscope by the use of 

 doublets and triplet*, the long course of the rays and the large angular 

 PI-IK il required in the compound instrument deterred the most sanguine 

 from anticipating the period when they should be conducted through 

 such a path free both from spherical and chromatic errors. 1'hilo- 

 sophcrs of no less eminence than M. Biot and Dr. Wollaston had 

 predicted that the compound would never rival the simple microscope, 

 and that the idea of achromatizing its object-glass was hopeless 

 can these opinions be wondered at when we consider how many years 

 the achromatic telescope had existed without an attempt to apply its 

 principles to the compound microscope. When we consider the small- 

 ness of the pencil required by the telescope, and the enormous increase 

 of difficulty attending every enlargement of the pencil when we con- 

 sider further that these difficulties had to be contended with and 

 removed by operations on portion* of glass so small that they arc 

 themselves almost microscopic objects, we shall not be mirpri.-i 

 even a cautious philosopher and most able manipulator like Dr. 

 Woll.iHt.in should prescribe limits to improvement. 



Fortunately for science, and especially for the departments of . 

 and vegetable physiqlogy, these predictions have been shown to be 

 unfounded. The compound microscope has been elevated from the 

 condition we have described to that of being the most imp 

 instrument ever bestowed by art upon the investigator of nature. It 

 now holds a very high rank among philosophical implements, while the 

 transcendent beauties of form, colour, and organisation which it reveals 

 to us in the minute works of nature, render it subservient to the most 

 delightful and instructive pursuit*. To these claims on our att. 

 it appears likely to add a third of still higher importance. The 

 microscopic examination of the blood and other human organic 

 will in all probab lity afford more satisfactory and conclusive ev 

 regarding the nature and seat of disease than any hitherto appealed to, 

 and will of consequence lead to similar certainty in the choice and 

 application of remedies. 



Soon after the year 1820 a series of experiments was begun in I'r.inc < 

 by M. Selligues, which were followed up by Frauenhofer at Munich, 

 by Amici at Modcua, by M. Chevalier at Paris, and by the Lite Mr. 

 Tulley in London. In 1824 the last-named excellent artixt, without 

 knowing what had been done on the Continent, made the attempt to 

 construct an achromatic objective for a compound microscop 

 produced one of nine tenths of an inch focal length, composed <.f 

 lenses, and transmitting a pencil of eighteen degrees. This was the 

 first that had been made in England ; and it is due to Mr. Tulley to 

 say, that as regards accurate correction throughout the field, that glass 

 has not been excelled by any subsequent combination of three lenses. 

 Such an angular pencil, and such a focal length, would bear an r\v 

 piece adapted to produce a gross magnifying power of one limxln-l .MM I 

 twenty. Mr. Tulley afterwards made a combination to be placed in 

 front of the first mentioned, which increased the angle of the trans- 

 mitt. d pencil to thirty-eight degrees, and bore a power of three 

 hundred. 



While these practical investigations were in progress, the subject of 

 achromatism engaged the attention of some of. the most proi'.iun.l 

 mathematicians in Knghiml. Sir John ilerschel, Professor Airy, 

 Professor Barlow, Mr. ('. .iMington, and others, contributed largely to 

 the theoretical examination of the Kubject ; and though the results of 

 their labours were not immediately applicable to the microscope, tln-v 

 ewentially promoted its improvement 



For sometime prior to 1S21) the subject had occupied the mind of a 

 gentleman, who, not entirely practical, like the first, nor purely 

 mathematical, like the lost-mentioned class of inquire, \\ns jeil to the 

 discovery of certain properties in achromatic coml.in.itin which had 

 been before unobserved. These were afterwards experimentally 

 verified ; and in the year 1829 a paper on the subject, by tii 

 Mr. Joseph Jackson Lister, was rend ami pui-li i ltoy.il 



Society. The principles and results thus obtained enabled Mr. 

 to form a combination of lenses which transmitted a pencil .; 

 degrees, with a large field correct in every part .- as this pa|wr v, 

 |.-uii.| ition of the recent improvement* in achromatic microscopes, and 

 as it* results are indispensable to all who would make <n 

 the mctrument, we A>-.\\\ ore impoitant parts of it in detail, 



and in Mr. l,i:t. r':< own words. 



" I would premise that the piano concave form for the correcting 

 flint lens has in that quality a strong recommendation, particularly as 

 it obviates the danger of error which otherwise cxinta in centring the. 

 two curves, and thereby admits of correct workmanship for a shorter 



