MICROCOSMIC SALT. 



MICROSCOPE. 



sti 



meat of nostitatiiig parallel bain. Such an inttraOMnt embodies the 

 principle of the b*t and most recent microoMter*. The most difficult 

 as well a* impnrtant part of this initrument U the acrew or screw* by 

 which the fork* are moved. Their thread* must not only be at the 

 aw HjtUmrt from each other, but have their inclination equal all 

 round. In the acre* of Troughton s micrometer there are about 10S 

 Illlsaili in an inch. 



The unual method of finding the value of a revolution of the screw 

 ha* been already given. The method of Professor Qaua* of Oottingen 

 U thus stated by Sir David Brewster (' Encyclopaedia Britannica,' art. 

 Micrometer ') : " He employ* for this purpose a standard telescope, 

 with a micrometer, the value of whoee Male hat been accurately deter- 

 mined. Since the wire* of a telescope-micrometer adjusted to distinct 

 vuion of the stow or planet* are accurately in the focu* of parallel ray* 

 falling on the objeet-gUat, it follow* that ray* issuing from the wire* 

 and falling on the inaide of the object glass will emerge from it perfectly 

 parallel. Now U we place the object-glass of the standard telescope 

 dose or new to that of the first telescope, the parallel ray* formed by , 

 those issuing from ita wire* will be refracted to the focus of the 

 standard telescope, and a distinct image of the wires will be there 

 formed. The observer therefore when he looks into the standard 

 telescope will see distinctly the wires of the first telescope, and by 

 mean* of hi* micrometer he will be able to measure exactly the angular 

 distance of these wires at whatever distance they may happen to be 

 placed. This angular distance, divided by the revolutions and parts 

 of a revolution which are necessary to bring the wires of the first 

 telescope to the zero of their scale, will give the value of one revolution 

 of the screw, or of one unit of the scale." 



The parallel fibres of a micrometer must be extremely fine, ami of 

 uniform diameter. Since Hooke's time, wires and fibres of silk have 

 been used instead of hairs. Troughton introduced the spider's line, 

 which he found to be sufficiently fine, ojMique, and elastic to answer 

 the purpoae : the stretcher, or long line which sustains the web, being 

 alone adapted. The fibres of spun glass, of melted sealing-wax, of 

 asbestos, and the minute crystals of mezzolite have also been used ; but 

 Wollaston's fine platinum wire [DUCTILITY] has of late years been 

 preferred. These wires offer another advantage in the method of illu- 

 minating them, which in the transit instrument is generally done by 

 means of a lamp, the light of which passes along the horizontal axis to a 

 small speculum which reflects it upon the wire. The new method is to 

 make the wires red hot by transmitting a voltaic current through them. 

 Arago (who first adopted, if he did not first suggest the idea) feared 

 that the images of objects beyond aud apparently near the heated wire 

 would undulate, but such was not found to be the case. This method 

 of illumination bos been used for the wires of the microscopic micro- 

 meters attached to the Oxford heliometer [HKLIOHETEII]. M. Porro's 

 mode of illuminating the wires is to mount them on glass plates, the 

 edges of which are formed into suitable curves, so as to introduce a 

 thin vein of light perpendicularly to the axis of the telescope, so that 

 the wires appear of a silvery white in a dark field. Brewster also pro- 

 posed to obtain micrometer lines by placing in the field of view the 

 reflected images of a fixed or moveable system of wires, or of lucid 

 discs, attached to the side of the eye-piece. The same distinguished 

 optician has also proposed what he colls jihutographic micrometer*. The 

 reader i* of cuurao aware of the ease with which microscopic photo- 

 graphs can be impressed upon a film of collodion as many as 10,000 

 portraits having been introduced into a square inch. " If a system of 

 opaque or transparent lines be impressed upon the collodion photo- 

 graphically, when reduced to the minutest size, from a system of large 

 and sharply defined lines, we shall have the most perfect tnicrometrical 

 cale that can be conceived, the portion of the collodion that contains 

 no nitrate of silver being as transparent as if the dark spaces were solid 

 wire* or metallic plates, placed in the focus of the eye-glass." 



The plan of opening and shutting a pair of parallel wires optically 

 mttmrt of mechanically, and of using it as a general principle in micro- 

 meters, has also been applied ; but for the details we must refer to the 

 treatise above quoted, in which will also be found minute details 

 respecting various forms of double-image micrometers, in which two 

 ugly refracting lrne, semi-lenses, or prisms, are separated by screws, 

 or where the two image* are separated optically, or in which the two 

 imago are formed by double refraction, and many others. 



For a description of micrometers for microscopes see MICROSCOPE. 



MICROC-OSMIC SALT (NaO, NH.O, HO, PO,). The tribasic 

 phosphate of aoda, water, and ammonia ; a salt much used as a flux in 

 blowpipe experiment*. [Bl-OwrilT..] 



MICROSCOPE, the name { an instrument for enabling the eye to 

 sea distinctly object* which are placed at a very abort distance from it, 

 or to we magnified image* of small objects, and therefore to see smaller 

 object* than would otherwise be visible. The name is derived from 

 the two Greek words, expressing this property, fu*p)>t, mall, and 

 *>, to lee. 



80 little is known of the early history of the microscope, and so 

 certain is it that the magnifying power of lense* must have been dis- 

 cover *J a* soon a* lease* were made, that there i* no reason for hazard- 

 in*; any doubtful speculation* on the question of discovery. We shall 

 proceed therefore at once to describe the simplest forms of micro- 

 copes, to explain their later and more important improvements, an 1 

 finally to exhibit the instrument in it* present perfect state. 



In doing this we shall assume that the reader is familiar with the 

 information contained in the articles LIGHT, LENS, ACHROMATIC, ABER- 

 RATION, and the other subdivisions of the science of Optics which are 

 treated of in this work. 



The use of the term magnifying has led many into a misconception of 

 the nature of the effect produced by convex lense*. It is not always 

 understood that the so-called magnifying power of a lens applied to the 

 eye, as in a microscope, is derived from its enabling tbte eye to approach 

 more nearly to it* object than would otherwise be compatible with 

 distinct vuion. The common occurrence of walking across the street 

 to read a bill U in fact magnifying the bill by approach ; anil tin- 

 observer, at every step he takes, makes a change in the optical arrange- 

 ment of his eye, to adapt it to the lessening distance between himself 

 and the object of his inquiry. This power of spontaneous ailjii 

 is so unconsciously exerted, that unless the attention be called to it by 

 circumstance*, we are totally unaware of it* exercise. 



In the case just mentioned the bill would be read with eyes in a very 

 different state of adjustment from that in which it was discovered on 

 the opposite side of the street, but no conviction of this fact would be 

 impressed upon the mind. If however the supposed individual should 

 perceive on some part of the paper a small speck, which he suspect* to 

 be a minute insect, and if he should attempt a very close approach of 

 his eye for the purpose of verifying his suspicion, he would presently 

 find that the power of natural adjustment has a limit ; for when bin 

 eye has arrived within about ten inches, he will discover that a further 

 approach produces only confusion. But if, as be continues to approach, 

 he were to place before his eye a series of properly arranged convex 

 lense* he would see the object gradually and distinctly increase in 

 apparent size by the mere continuance of the operation of approaching. 

 Yet the glosses applied to the eye during the approach from ten inches 

 to one inch, would have done nothing more than had been previously 

 done by the eye itself during the approach from fifty feet to one foot. 

 In both cases the magnifying is effected really by the approach, the 

 lenses merely rendering the latter periods of the approach compatible 

 with distinct vision. 



A very striking proof of this statement may be obtained by the 

 following simple and instructive experiment. Take any minute object, 

 a very small insect for instance, held on a pin or gummed to a slip of 

 glass ; then present it to a strong light, and look at it through the finest 

 needle-hole in a blackened cord placed about an inch before it The 

 insect will appear quite distinct, and about ten times larger than its 

 usual size. Then suddenly withdraw the card without disturbing the 

 object, which will instantly become indistinct and nearly in. 

 The reason is, that the naked eye cannot see at so small a distance as 

 one inch. But the card with the bole having enabled the eye to 

 approach within an inch, and to see distinctly at that distance, is thus 

 proved to be as decidedly a magnifying instrument aa any lens or com- 

 bination of lenses. 



This description of magnifying power does not apply to such instru- 

 ments as the solar or gas microscope, by which we look not at the 

 object itself, but at its shadow or picture on the wall ; and the descrip- 

 tion will require some modification in treating of the compound 

 microscoix), where, as in the telescope, an image or picture is i 

 by one lens, that image or picture being viewed as an original object 

 by another lens. 



It is nevertheless so important to obtain a clear notion of the real 

 nature of the effect produced by a lens applied to the eye, that we will 

 adduce the instance of spectacles to render the point more familiar. 

 If the person who has been supposed to cross the street for the purpose 

 of reading a bill had been aged, the limit to the power of adjustim-nt 

 would have been discovered at a greater distance, and without so 

 severe a test OH the supposed insect. The eves of the very aged gene- 

 rally lose the power of adjustment at a distance of thirty IT 

 inches instead of ten, and the spectacles worn in consequence are as 

 much magnifying glasses to them as the lenses employed by younger 

 eyes to examine the most minute objects. Spectacles are magnifying 

 glnnsfm to the aged because they enable such persons to see as closely 

 to their objects as the young, and therefore to see the objects larger 

 than they could themselves otherwise see them, but not larger than 

 they are seen by the unassisted younger eye. 



In saying that on object appears larger at one time, or to one 

 person, than another, it is necessary to guard against misconception. 

 By the apparent size of on object we mean the angle it subtends at the 

 eye, or the angle formed by two lines drawn from the centre of the 

 eye to the extremities of the object. In jiy. 1, the lines A and B E 



drawn from the arrow to the eye form the angle A E B, which, when 

 the angle i* small, is nearly twice aa great as the angle c E D formed by 



