MICROSCOPE. 



MICROSCOPE. 



which will be shortly noticed. This wa nrrt accomplished in a satis- 

 factory manner by 



Dr. WuUatto* 1 ! doMti, 



invented by the oelebrmted philotopher wboee name it beam; It ooniUU 

 of two planoconvex lenee* (fig. 4) having their focal 

 " * length* in the proportion of 1 to 8, or nearly so, and 

 placed at a distance which can be ascertained best by 



actual experiment. Their plane sides are placed toward* 

 the object, and the leu of shortest focal length next 

 _ the object. 



It appear* that Dr. Woll.wton wag led to thin invention 

 by cowMermg that the achromatic Huyghenean eye-piece, which will be 

 hereafter described, would, if reversed, possess similar good properties 

 a* a simple microscope. But it will be evident when the eye-piece la 

 understood, that the circtunstancea which render it Achromatic are 

 very imperfectly applicable to the simple microscope, nnd that the 

 doublet, without a nice adjustment of the stop, would be v;i! 



Uaaton makes no allusion to a stop, nor is it certain that ho 

 contemplated it introduction, although his illness, which tenn 

 fatally soou after the presentation of his paper, may account for the 



: - 



The nature of the corrections which toko place in the doublet is 

 explained in the annexed diagram (Jiff. 5), where L o L' id the object, 

 ! a portion of the pupil, and o D the stop, or limiting aperture. 



I 



L O I/ 



Now, K will be observed that each of the pencils of light from the 

 extremities L L' of the object is rendered excentrical by the stop, and 

 of consequence each passes through the two lenses on opposite sides of 

 their common axis o r ; thus each becomes affected by opposite errors. 

 which to some extent balance and correct each other. To take the 

 pencil L, for instance, which enters the eye at R u R B, it is bent to the 

 right at the first lens, and to the left at the second ; and as each 

 bending alters the direction of the blue rays more than the red, and 

 moreorer as the blue rays fall nearer the margin of the second lens, 

 where the refraction, being more powerful than near the centre, com- 

 pensates hi some degree for the greater focal length of the eeond ],-,,. 

 the blue and red rays will emerge very in 



sequence colourless to the eye. At the same time the Kphcricnl aber- 

 ration has been diminished by the circumstance that the side of the 

 one lens nearest the axis passes the other nearest 



; 



.1 . , i. 



. 



This explanation applies only to the pencil) near the extremities of 

 the object The central pencil, it is obvious, would pan both lenses 

 symmetrically ; the aune portions of light occupying nearly the same 

 relative places on both lenses. The blue light would enter the second 

 lens nearer to it* axis than the red, and being thus less refracted than 

 the red by the second lens, a small amount of compensation would take 

 place, qnite dlffneut hi principle and inferior in degree to that 

 produced in the excentrical pencils. In the intermediate spaces the 

 corrections are still more imperfect and uncertain ; nml this explains 

 the cease of the aberrations which must of necessity exist even in the 

 beet-mad* doublet. It is, however, infinitely nu|>eru>r to a sing 

 and will transmit a pencil of an angle of from 85* to 50* without any 

 very sensible errors. It exhibits therefore many of the usual test- 

 objects in a very beautiful manner. % 



The next step in the improvement of the simple microscope bears 

 more analogy to the eye-piece. This Improvement was mode >>y Mr. 



Holland, and it consists (as shown in fy. 6) in substituting two 1 

 for the first in the doublet, and retaining the stop j..j_ 



between them and the third. The first bending, 

 being thus effected by two lenses instead of one, is 

 accompanied by smaller aberrations, which are there- 

 fore more completely balanced or corrected at the 

 second bending, in the opposite direction, In 

 third lens. This combination, though colled a triplet, is essentially a 

 doublet, in which the anterior lens is divided into two. For it must 

 be recollected that the first pair of lenses merely accomplishes u li.it 

 might have been done, though with lees precision, by one ; but the 

 two lenses of the doublet are opposed to each other; the second 

 diminishing the magnifying power of the first. The first pair of lenses 

 in the triplet concur in producing a certain amount of magnifying 

 power, which is diminished in quantity and corrected as to aberration 

 : tli. thirl lens l>\- the change in relation to the position of the axis 

 which takes place in the pencil between what is virtually the first and 

 second lens. In this combination tin- errors are ntill further >' 

 by the close approximation to the object whit 1 



to take place near the axis. Thus the tranm larger 



angular pencil, namely 65, is rendered compatible with 'distinctness, 

 and a more intense image is presented to the eye. 



Kvi-ry increase in the number of lenses is attended with one draw- 

 back, from the circumstance that a certain portion Hi li^-lit is lost by 

 reflection and absorption each time that the ray enters a new medium. 

 This loss bears no sensible proportion to the gain arising from the 

 increased aperture, which, being as the square of tl, 

 multiplies rapidly; or if we estimate by the angle of the admitted 

 pencil, which is more easily ascertained, the intensity will be as the 

 square of twice the tangent of half the angle. 

 To explain this, let D B (./?;/. 7) represent the 

 diameter of the lens, or of that part of it which 

 is really employed ; c A the perpendicular drawn 

 from its centre, and A B, AD, the extreme rays of 

 the incident pencil of light DAB. Then the 

 diameter being 2 CB, the area to which the in- 

 tensity of vision is proportional will be (2 c B) S , 

 and c B is evidently the tangent of the angle CAB, 

 which is half the angle of the admitted pencil 

 DAB. Or, if a be used to denote the angular 

 aperture, the expression for the intensity is 

 (2 tan. 4 a) 5 , which increases BO rapidly with the 

 increase of a, as to make the loss of light by re- 

 flection and absorption of little consequence. 



The combination of three lenses approaches, as has boon 

 very close to the object; so close indeed as to pr use of 



more than three;, and this constitutes a limit to the improvement 

 of the simple microscope, (for it is called a simple microscope, although 

 consisting of three lenses, and although a .compound microscope may 

 be made of only three or even two lenses;) but the different ;n > 

 ment which gives rise to the term compound will be better 

 stood when that instrument is explained. 



Before we proceed to describe the simple microscope and its 

 appendages, it will be well to explain such other points in reference to 

 the form and materials of lenses as are most likely to be interesting. 



A very useful form of lens was proposed by Dr. Wollaston, and 

 called by him the periscopic lens. It consisted of 

 two hemispherical lenses, cemented together by their 

 plane faces, having a stop between them to limit 

 the 'aperture. A similar proposal was made by Kir. 

 '..Islington, who, however, executed the proj 

 a better manner, by cutting a groove iu a whole 

 sphere, and filling the groove with opaque i. 

 His lens, which is the well-known Coadingtoo 

 .vn in fiy. 8. It gives a large field of 

 which is equally good iu all directions, as it i.i 

 evident that the pencils AA' and B B' pass tli 

 under juvcisely the KIH. 



Tin has the fun! i,'a of ren 



the position in which it is held of comparatively 

 little consequence. It is therefore very convenient 

 as a hand-lent, l>ut its definition is of course not so 

 good as that of a well-made doublet or achromatic 

 [ ;. . 



Another very useful form of doublet was proposed 

 liy Sir John Herschel, chiefly like the CoddinKlon 

 r ill.- sake of a wide field, and chiefly to be 

 used iu the hand. It is shown in Ji;i. 9 : it consists 

 of a double convex or croased I.TIH. h.ivin^l! 

 of curvature as 1 to 6, anil of a p 1 7t lens 



whose focal length U to that of the convex lens as 

 13 to 5. 



udeed innumerable, other forms and c..inl.inat.ions of 

 lenses have l>een projected, co; 14 much ii 



of any practical use. In the Cat.adioi.tric. leii-cd the \IL 

 at right angles from its entering direction, )>cuig re, 

 a surface cut at an angle of 45 degrees to the axes of the 

 surfaces. 



