222 



MICROSCOPE. 



Sir Isaac 

 Newton's 

 reflecting 

 micro- 

 scope, 

 1678. 



Dr. Bar- 



Utr's te- 



scopes which he used. One of these was composed of 

 a n eye-glass, a middle-glass, and an object-glass, 

 mounted in a cylindrical tube placed horizontally. 

 Behind the stage was a small tube, with a convex lens 

 at each end, and a lamp beyond it whose light was 

 concentrated by the lenses in the tube, and thrown 

 upon the object to be examined. The instrument pos- 

 sessed various adjustments, and was regulated by a 

 rack and pinion. 



Sir Isaac Newton seems to have been the first person 

 who suggested the use of a reflecting microscope. He 

 placed the objects before a concave speculum, so that an 

 enlarged image of them was formed at a greater dis- 

 tance. This image being viewed with a convex eye- 

 glass, was again magnified, and appeared very distinct. 

 The great defect of this instrument arose from its being 

 inapplicable to opaque objects, in consequence of the 

 objects being placed between the object speculum and the 

 eye-glass. See the Phil. Trans. H>72, No. 80, p. 3075, 

 and the Appendix to Gregory's Elements of Optics. 



Another form of the reflecting microscope was sug- 

 gested by Dr. Robert Barker, in the year 1736. It was 

 fleeting mi- nothing more than the Gregorian telescope converted 

 croscope. into a microscope, merely by lengthing the tube, and 

 therefore requires no particular description. There 

 can be no doubt that it would give very distinct images, 

 and has the advantage of allowing the light to fall freely 

 upon the object, which is placed at a distance of from 

 9 to 24 inches beyond the tube. See Phil. Trans. 1736, 

 Vol. 3y. No. 442, p. 25.9. 



Dr. Robert The next compound microscope was invented by 

 Smith's re- Dr. Robert Smith, Professor of Experimental Philosophy 

 tlecting mi- at Cambridge. Having had occasion to examine the 

 croscope, principles of Sir Isaac Newton's reflecting microscope, 

 he constructed one of them, in which the foca' distance 

 of the speculum was 2y inches. He " found that the co- 

 lours of objects appeared much more beautiful and 

 ' natural than in doubly refracting microscopes of the 

 ' best sort, their proper colours being free from the 

 ' mixture of other colours arising in refracting micru- 

 scopes, fromthedifferentrefrangibility of rays." Smith's 

 Optics, Vol 1. p. 279. He found also that objects ap- 

 peared sufficiently bright, and very distinct, when the 

 reflecting microscope had the following dimensions : 



Focal length of the speculum . . 2| inches. 



Diameter of the speculum ... 1 



Focal length of the plano-convex eye- 

 glass 2} 



Ratio of the distance of the object from the 

 focus of the speculum, to the focal dis- 

 tance of the speculum . . . 1 to 14 



Having found that, in order to produce a high mag- 

 nifying power with this reflecting microscope, it was 

 necessary to have the speculum very concave, and there- 

 fore very small, he set about contriving a microscope 

 with two reflecting spherical surfaces of any size, so 

 proportioned to each other that the aberration of the 

 rays, caused by the first reflection, should be perfectly 

 corrected by the second, and consequently that the 

 last image of the object from which the rays diverge 

 upon the eye-glass, shall be perfectly free from aber- 

 ration. 



One of Dr. Smith's microscopes is shewn in Fig. 



18. where A A is a concave spherical speculum, and 



1 ig. 19. c(j a convex spherical speculum, having its polished 



canvex surface inwards. The rays from an object 



1738. 



o placed in the slider mn, will be reflected from Compound 

 the concave speculum AA upon the concave CC, and .Micro- 

 will have a distinct and magnified image of it formed 

 before the convex eye-glass E, by which it will be ' 

 magnified still more. This instrument, in short, is no- 

 thing more 'than the Cassegrainian telescope convrrted 

 into a microscope, with this difference only, that, in 

 the telescope, distinct vision is obtained by moving the 

 convex mirror, whereas, in the microscope, it is obtained 

 by a motion of the eye-glass. Dr. Smith constructed 

 one of these microscopes, which he found to perform 

 " nearly as well, in all respects, as the very best re- 

 fracting microscopes ;" aiid the writer of this article 

 has one of them now before him, which performs won- 

 derfully well, though both the specula have their polish 

 considerably injured. The following are the dimen- 

 sions, &c. of Dr. Smith's reflecting microscope as given, 

 by himself: 



Focal length of both specula . 1.0000 



Distance of the centres of both specula 1.6558 



Distance of the image from the centre 



of the concave speculum . . 1.1337 



Focal length of the eye-glass . 0.1407 



Distance of the eye behind the eye- 

 glass 0.1479 



Diameter of the eye-hole . . 0.0190 



Distance of the object from the centre 



of the convex speculum . . 0.0626 



Length of the concave speculum . 15 4-7 



Arch of the convex speculum . 4 50' 44)" 



Distance of the stop s from the object 0.4545 



Diameter of the stop s . . 0.038 



Diameter of the hole in the concave 



speculum ..... 0.143 



Diameter of the hole in the convex 



speculum ..... 0.019 



Magnifying power, the focal length, 



&c. of the eye being 8 inches . 300 times. 



The stop s is placed behind the convex speculum, 

 in order to prevent the direct rays from the object, 

 which would pass unreflected through the openings in 

 both specula, and fall upon the eye-glass, from mixing 

 with the rays regularly reflected by the specula, and 

 forming the magnified image upon the retina. See 

 Smith's O/ilics, Vol. h. Remarks, p. 94. 



The next microscopes which exciled any notice, were Delcbarre* 

 those of M. Delebarre of the Hague, who began to con- ni|cr - 

 struct them in 1771. Montucla saw them when he 5C0 1 K ' 3- 

 travelled in Holland in 1773, and induced Delebarre 

 to go to Paris, where he was well received, and where he 

 sold many of his instruments, each of which cost about 

 15*. M. M. Montigny, Leroi, and Brisson, submitted 

 to the Academy, on the 21st of June 1777, a most flat- 

 tering report of their performance, and Montucla has 

 given a very full account of them in his History of Ma- 

 thematics. We have read these eulogiums and de- 

 scriptions with great attention, and are obliged to ac- 

 knowledge, that we cannot find in the microscope of 

 Delebarre any improvement of the least importance 

 which had not been known and adopted by the Lon- 

 don opticians. The only point of tlificrence that we 

 can observe is, that of changing' the rye-glasses, and 

 of combining six lenses at once, which Montucla semis 

 to lay considerable stress upon, because Euler in his 

 paper 7>e Aora Miproscomorum gtnerc, published in 

 the memoirs of St. Petersburgh for 1766 and 1767, had 





* Delebarre described his microscope in a memoir which was read at the Academy of Sciences in 1777, and in a separate pamphlet en- 

 titled Dcscriftion et F Usage du Mkroscofe Universe^ 



