180 



MICROPHONE 



MICROSCOPE 



which may net upon the surrounding air and 

 give rue to sonorous wave*; or the variations 

 in the current may le detected by tin- Tclf|ihnm> 

 i'|.v.i. One form of nknphoM coiisiis of a 

 piece of mercury -tem|>ered carUm, an inch long, 

 placed vertically hetween two cnrlK>n.l>li>rks 1ml 

 lowed to receive its ends : wire* connect the Mocks 

 with the luitteiv and with the receiver hy which 

 the sounds are to he heard. 'A piece of willow- 

 charcoal,' says the inventor, ' the si/e of a pin's 

 bead is siitlicicnt to reproduce articulate speech.' 

 Two nails laid |iarnllel, with wire conned ions, and 

 a third nail laid across them, make a simple form 

 of microphone. A lew cell- of any form of battery 

 may IK- used. Many useful applications of the 

 microphone have heen made or suggested. 



MicriiKrope (Gr. mikrot, 'small ; ' and skopeo, 

 'I see") is an instrument for enabling us to 

 examine objects which are so small as' to he 

 almost or i|iiite undtacernible liy the unaided 

 eye. It* early history is obscure"; hut, as it is 

 quite evident that the property of magnifying pos- 

 sessed by the lens must have \>een noticed a-, sooif 

 as it was made, we are quite safe in attributing 

 its existence in its simplest form to a period 

 considerably anterior to the time of Christ. It is 

 generally Mieved that the first compound micro- 

 scope was made hy Zacharias Jansen, a Dutchman, 

 in the year 1590, and was exhibited to .lamps I. 

 in London by his astronomer, Cornelius |ii.-l,Ue], 



in Hil'.t. It was then a very imi>ericct iiistru lit. 



colouring and distorting all objects. For many 

 years it wan more a toy than a useful instrument. 

 and it was not until the invention of the achromatic 

 lens by Chestermoor Hall ( 1720) and John Dollond 

 ( 17.V2-57), and its application to the microsco]ie by 

 Lister and others, that it reached .the advanced 

 position it now occupies among scientific in-tm 



nielli-. 



An object to lie magnified requires simply that 

 it be brought nearer to tlie eve than when lirst 

 \.iminc.l ; but as the focal distance of the eye 

 ranges from 6 inches to 14 inches 10 inches being 

 the average focal distance it follows that a limit to 



the magnifying power of tl ve is attained when- 



ever the object to IK- examined is brought too near. 

 If, however, MC blacken a card, anil pierce a hole in 

 it with a line needle, and then examine a minute 

 object, as, for instance, the wing of an insect held 

 aliout an inch from the card, we shall see it di- 

 tinctly, and that, too, magnified altonl ten times its 

 sire. This i- explained by the fact that the pin- 

 hole limits the divergence of the pencil of rays from 

 each |Miint of the object, so that the eye can con- 

 verge it .sufficiently on the retina to" produce a 

 distinct iinpre-sjiin, which is faint: and did not 

 the blackened i-aid exclude all other light it would 

 lie lost. If we now remove the blackened card 

 without either moving our eye or the object under 

 examination, it will IN- found "that I lie insect's wing 



is almost invisible, tl misstated eye licing un- 



able to sec clearly an object s<i near as one inch; 

 thus demons! ra! in;; the hlackcn.-d card with the 

 needle hole in it to lie as decided a magnifying 

 instrument as any - t of lenses. 



In fig. I Alt is a double convex lenn, in front of 

 which, hetween it and il focus. K. but near that 

 I.M-IIS, we have dniMii an arrow, EF. to represent 

 I he object under itis|iecti<in. The eones drawn fioni 

 its extreme (minis an- representative ray of light, 

 diverging from these points and falling on the lens. 



These rays, if not interrupt^! in their cimrw by tin- 

 lens, AH, would IK> too dhergcnt for I ho eve to 

 bring them to a focus n|>on the retina (we K\ i 



eye than the least distance of distinct vision, which 

 is, for most individuals, al>oul ten inches. Sup 



po-e the li-n- is as close as may lie to ll ye. and 



that the object. KF. is brought up to it to such a 

 distance that the virtual image. d>. is at Id inches' 

 distance from the eye; and let us further suppos. 

 that the focal length of the lens ta such (gee I.KNS) 



Hut after triivcr-ingthc lens, A 11. they travel, if the 

 object In- snlli. ienily near the focus, K, ill lines 

 which are nearly parallel, or which apparently 

 diverge from point-, such as C, D, not nearer to the 



Fig. I. 



that the image, CD, is ten times, linearly, as great 

 as EF ; then the eye, instead of vainly striving to 

 see the small object, EF, near K, will seem to ]>er- 

 ceive distinctly an image ten times as great 

 linearly, and situated at the convenient distance, 

 H. The magnification of the lens is independent 

 of the eye, and is the relation between the si/e of 

 the image and that of the object. \Vhen one of 

 these is at an infinite distance and the other at a 

 principal focus, the magnifying po\ver depends on 

 the position of the eye, and is the ratio bet u een 

 the apparent size of the object at any given dis- 

 tance and that of the virtual image as seen with 

 the aiil of the lens ; this may be seen to increase as 

 the rye is withdrawn to a greater distance, especi- 

 ally wl the one eye is used to look at il bject, 



sax a page of print, and the other to h>ok through 

 the lens ; hut the greatest retinal image is formed 

 when the lens is close to the eye. 



\Ve have supposed the whole of the light to enter 

 the eye through the lens, All (fig. 1 ) ; but so large 

 a |H'iicil of light passing through a single lens Mould 

 he so much distorted by its spln-iica! figure, and by 

 the chromatic dispersion of the gla-ss, as to p induce 

 a \ery indistinct and imperfect image. This j, 

 partly rectified by applying li slop to the lens, so 

 as to allow only the central portion of the pencil to 

 pass. Hut, while such a limited pencil Would repre- 

 sent correctly the form and colour of the object, so 

 small a pencil of light is generally unable to illu- 

 minate the whole of the magnified picture with any 

 adequate degiee of brilliancy, and is therefore in 

 capable of displaying I hose organic markings on 

 animals or plants which are often of so much im- 

 portance in distinguishing one class of objects from 

 another. !>r \\ollaston was the first to overcome 

 this difficulty, which he achieved by constructing 

 n doublet (fig. 2), which consists of 



two phii nvex lenses, having their 



focal lengths in the proportion of 1 



to .'I. and placed at a distance best 



asceitaincd by ex|'riment. Their 



plane sides are placed towards the 



object, and the lens of shortest local 



length next the object. Hy this arrangement the 



distortion caused by the first lens is corrected by 



the second, and a well defined and illuminated 



image is seen. Dr XVollnston's doublet w as further 



improved by Mr Holland, who substituted two 



lenses for the first in Dr \Volla- 



ston's doublet, and retained the 



slop bctMcen them and the third. 



This combination, though generally 



called a triplet, ta virtually a 



doublet, inasmuch as the two lenses 



only accomplish what the anteiior 



Fig. 3. 



lens did, although with less precision, in Dr Wolla- 

 ston's doublet. In this combination (fig. 3) of 



