I C R O M E T E R. 



Micro 



trical 



Ijsfer. 



PliT 



of its power is the same A micrometer with a divided 

 jr eye-glass, may profess to measure as far as TT i 55 of 

 an inch; but the next division is T -:'-- or rtfis- '* 

 and though the eye may be able to distinguish that 

 the truth lies between the two, it receives no assistance 

 within part of the larger measure." I'lul. 'front. 

 1813. p. 11<). 



In tins ingenious instrument, where the variation 

 of the scale is produced by placing the scale of wires 

 at different distances from the oW-r\er, it is necessary 

 that the scale be viewed by the naked eye, which 

 by its adapting power can command <i :on of 



it at different distances. We conceive, however, that 

 it would be of advantage to have the scale at a per- 

 manent distance, and to view both the scale and the 

 object to be measured with the aid of a lens, which 

 may be easily done by placing a lens UJKHI the small 

 perforation. In this cast- it would be necessary to ob- 

 tain a variation of the scale by means entirely different 

 li-oin those adopted by Dr. Wollaston. 



A micrometer founded u|>on the-? principles has 

 been proposed by Dr. Brewster, and may be easily con- 

 structed by a slight addition to a good kaleidoscope. 

 ILXXT[ Let ABC be the aperture of the reflectors, and let a b, 

 f t , n_ tie be the portion of a win- , th of an inch in diame- 

 ter, that is seen directly through the aperture. U hen 

 the wire is perpendicular to die reflector BC, its image 

 lie, r/'will coincide with the wire iulf, and form an 

 exact continuation of it. If we now turn round the 

 cap till the wire comes into the po.it mn a' tf, d' t', its re- 

 flected image t/ 1?, eV' will be inclined to the wire, and 

 the thickness bft', or the oblique section ,.i tin- wire, 

 will be to its thickness b t or ,'- th of an inch as the 

 secant of the arch (A) through which the wire has 

 pasifd in going from the position a b, d e into the posi- 

 tion * tf, f, is to 1 , that is b' e'=b e x sec. A= ,' x 

 sec. A. In this way the thickness of any wire may be 

 nude to vary and form a scale by which any other ob- 

 ject may be measured by |>rjr>tion against it, if it In- 

 very minute, and by juxta position if it is nearly ot the 

 same sue. The unit of the scale, or abdr, may be 

 either a wire as already mentioned, or a luminous aper- 

 ture, or the interval Ix-tween two wires, and in order 

 that the arch A may I*- always small, in which case 

 the secants vary slowly, differ r standards may 



be fixed upon the can of the instrument, whit h i- di- 

 vided into degrees for the purpose of measuring the 

 arch A. 



W hen the object is very minute, it must be placed 

 in the focus of a magnifying lens, while the eye looks, at 

 the scale cither through a perforation in that leu 



has a considerable focal length, or at its side, if it has 

 a small focal length. If the object is not microscopical, 

 as ABCD, it may be measured by introducing it at the 

 object end of the instrument, and having raised the 

 standard scale amn d till its oblique section m n is ex- 

 actly e<|ual to the diameter of the object ABCD. It is 

 unnecessary to enter into any farther details respecting 

 this instrument, as our only object at present is to ex- 

 plain the principle on which it is founded. 



A similar instrument may be made by means of a 

 doubly refracting plate placed at the upper end of a 

 tube. A rectangular aperture is placed before this 

 plate, of such a magnitude that its two images are ex- 

 actly in contact when they are parallel to each other. 

 By giving a rotatory motion of 90 to the rectangular 

 aperture, the two apertures coincide in the direction of 

 tlieir length, and we have an arch of 90 to measure 

 the variation. The breadth of the compound aperture 

 will always be A-f-A sin. 9, A In-ingthe breadth of the 

 rectangular aperture and f> the arch described by the 

 aperture reckoned from the longitudinal coincidence of 

 the two images. 



The principle of double refraction may be advanta- 

 geously employ rd in the construction ofamicnnnetM tor 

 measuring directly the magnitudes of objects of extreme 

 minuteness. Let it be required, for example, to measure 

 N whose diameters are between the r .' t th and r V^th 

 of an inch. Take a plate of rock crystal AB, CD about 

 -' inches long, whose thickness AB at one end is ten 

 time* greater than its thickness CD at the other, and 

 let the plate be cut out of the crystal, so that the two 

 images of a wire T J c th of an inch in diameter will be in 

 accurate contact, then at the other end (I), the two 

 image* of a wire r -Vvth of an inch in diameter will lie 

 in accurate contact, and wires, or objects of all inter- 

 mediate magnitude*, w ill be measured by observing tin- 

 part of the scale between A and C where their two 

 images are in contact. This scale being two inrlu , 

 long, may consist of a plate of mother-of-pearl attached 

 to the rock crystal, and divided into 400 parts, so that 

 each division will correspond to the *J or a little more 

 than T ^i Jjdth parts of an inch. A prism of crown 

 glass is cemented to the prii-m of rock crystal, in order 

 to correct the dispersion, and a lens of high magnifying 

 power U made to slide along it, in order to observe the 

 coincidence of the images. The principle explained in 

 the preceding paragraph may also be employed alone 

 in plates of rock crystal, the object to be measured be- 

 ing substituted in place of the rectangular aperture. 

 Or both these principles may be combined in one iu- 



On Mi. i,.- 

 meters foi 

 Micro- 

 scopes. 



Microsfi.- 

 plcal mi- 

 crometer 

 by double 

 refraction. 



Kg. 14. 



M .<">- 



MICROSCOPE. 



H I r-noscopE, from !((, imall, and ntwui, to ter, is 

 the name of a well known optical instrument, for seeing 

 or magifying minute objects. 



Tlii .< microscope, like that of the te- 



lescope, ha* been claimed for more than one indivi- 

 dual * !' t. r Borrllui, in his work Dr >CODII 

 fMCft/orr, published at the Hague in Kj.V., has addu- 

 ced a good deal of evidence connected w ith the inven- 



tion of the telescope and thi 

 wan! five different tertimon 

 Boreel, envoy from i 

 considerable light on the sv 

 acquainted with Za> 

 been in his father'- shop. 



' microscope. He brings lor- 

 ies, and a Icttt r from William 

 'I. which throw 

 biect. . Boreel was intimately 

 Jansen, and had frequently 

 He had often heard that the 



Jansens were the inventors of the microscope ; and hav- 

 ing been in England in 1619, he saw in the hands of In- 



with the use of the single microscope, si Irut in one of its form*, is in. 



tt 4&MMBW * vjim*m~ " __ - * -- 



rrr- 



p. / . 



Xat.qtat, lib. i. 



