MICROSCOPE. 



653 



paedia, the invention, theory, use, and various char- 

 acter of this instrument, are perspicuously though 

 briefly stated in popular terms, suitable to the na- 

 ture of the work. At the period of its insertion 

 a new era had lately opened in the history and ap- 

 plication of the microscope ; much had been done 

 and was still doing to improve the instrument, in 

 the way of enlarging its powers and correcting its 

 eccentricities ; and great accessions were daily 

 being made to our knowledge of minute animals 

 and formations. These, however, were not suffi- 

 cient reasons for departing from the general plan 

 of this Encyclopaedia, by giving remarkable pro- 

 minence to the article Microscope at that time, 

 the more especially as it was already obvious that 

 a supplementary volume would be necessary, where- 

 in this and other subjects might receive the degree 

 of attention which their importance should deserve. 

 It has therefore been reserved for the present time, 

 to give a more full and satisfactory detail of the 

 improvements effected on the instrument, (chiefly 

 by the labours of Wollaston, Brewster, Herschel, 

 Pritchard, Goring, Coddington, Gary, &c.,) and 

 of the interesting phenomena which, in consequence 

 of these improvements, have been laid open to our 



Avoiding as much as possible a repetition of the 

 former article, and an iteration of the particulars 

 comprehended under the heads Optics, Lens, &c., 

 it is still desirable to bring the first principles of 

 the microscope into close connection with all their 

 subsequent adaptations and combinations, and this 

 we shall do in as few words as possible. 



An object is rendered visible by the light re- 

 flected from its surface, and the eye perceives it by 

 cones of rays issuing from every point thereof, 

 the apex of those cones being the points whence 

 they are emitted, and their common base the pupil 

 of the eye. Now, though the eye can to a certain 

 extent contract or dilate the pupil, and otherwise 

 adapt itself to the rays falling upon its surface, it 

 is still found from experience that no rays will be 

 converged on the retina, (see Retina} and conse- 

 quently no distinct image of the point whence they 

 proceed be impressed upon it, unless those rays ap- 

 proach very nearly to parallelism. It must there- 

 fore be obvious that there is a certain distance at 

 which an object should be placed from the eye, in 

 order to be seen distinctly, or, in other words, that 

 the sides of the radiant cones may approach very 

 nearly to parallel lines. This requisite distance for 

 distinct vision varies considerably in different per- 

 sons ; scene who are exceedingly short-sighted, that 

 is, who have an undue convexity of the eye, re- 

 quire the object to be brought within two inches 

 of the visual organ, whilst others whose eyes have 

 less convexity than is usual, can see most con- 

 veniently at the distance of ten inches, or perhaps 

 more. Now, bearing in mind that an object cannot 

 be brought nearer to any eye than a certain point, 

 without creating confusion and indistinctness in the 

 image conveyed to the retina, and also considering 

 that to iiiiignify an object, is only in other words 

 to bring it within that distance, it becomes evident 

 that if we obtain means for procuring distinct vision 

 at any point nearer to the eye than the natural 

 focus, we at once enlarge the images of objects in 

 an inverse ratio of the two distances. To esti- 

 mate in a popular manner the magnifying powers 

 of microscopes, it has always been necessary to as- 

 sume some standard distance for distinct unaided 

 vision ; though, from the circumstance of scarcely 



any two individuals having precisely the same focus 

 of natural sight, the calculations referred to such 

 standard are less satisfactory than could be wished. 

 The early micrographers adopted six or eight 

 inches as a fair average, and, in the present day, 

 Sir D. Brewster limits distinct vision to five inches, 

 whilst Mr Pritchard extends it to ten inches. Of 

 all these standards, that of eight inches approaches 

 most nearly to the general focus of the human eye; 

 but none of them is free from a multitude of ex- 

 ceptions. Perhaps Mr Pritchard's standard of ten 

 inches is the best, as it offers all the conveni- 

 ences of decimal notation; but in fixing such a 

 focus for the natural eye, it should be premised that, 

 without reference to the actual focus of any eye, 

 the object is to be viewed by it at the distance of 

 ten inches, in order that the ratio on which the 

 magnifying power of the microscope is calculated, 

 may universally hold good. Persons who from 

 short-sightedness cannot see an object at ten inches, 

 must determine the magnifying power expressly for 

 themselves. Dr Goring, indeed, advises that the 

 expression of the magnifying power should be dis- 

 carded altogether, and that we should simply state 

 the focal distance of the magnifier, leaving to every 

 person to determine the correct ratio between it 

 and the focus of his own eye. 



The first rudiments of a microscope, and the first 

 efforts at microscopical observation, are to be found 

 in the nursery. A child perforates a card with a 

 pin, and looking upon his book through the small 

 aperture, at a distance of two or three inches, he 

 sees the letters greatly magnified. In this case the 

 aperture becomes the base of the radiant cone, in 

 place of the pupil of the eye, and though the 

 quantity of light is considerably diminished, the 

 parallelism of the two sides of the cone produces 

 distinct vision. This is of course only mentioned 

 in explanation of a principle ; for a small vacant 

 aperture merely producing a limited effect by di- 

 minishing the light, can answer no useful practical 

 purpose. It is to Refraction, (qu. vide) or the 

 bending of the rays of light by the intervention of 

 some transparent medium between the eye and the 

 object, or to Reflection (qu. vide) by means of 

 concave specula, (see Speculum') that we must look 

 for a satisfactory magnifying power. 



It is found that a circular piece of glass, one or 

 both of whose surfaces are convex, (see Zens) re- 

 fracts or bends the rays of light towards the centre 

 of its convexity ; and that if an object be placed 

 in its focus, (which may be determined practically 

 by observing at what distance it will concentrate 

 the solar rays,) the sides of the cones of rays pro- 

 ceeding from all the points in that object are 

 rendered parallel after transmission ; consequently 

 an eye on the other side of the lens will perceive 

 the object distinctly at its focal distance, whatever 

 that may be, and will see it magnified in the ratio 

 of the focus of the lens to the natural focus of the 

 eye. Regarded as magnifying powers, concave 

 specula and convex lenses are the counterpart, of 

 each other ; the reflection of the one producing 

 the same effect in the enlargement of the object, 

 as the refraction of the other. 



Microscopes are either single, or double (or com, 

 pound;) the former consist of a single lens or spe- 

 culum, the latter of a combination of lenses, or of 

 lenses and specula. They are likewise distinguished 

 into refracting, reflecting, and refracting-reflecting 

 microscopes ; in the first case they are formed of 

 lenses, in the second of specula, and in the third 



