1- 



M I C R O S C P E. 



221 



Single Mi- 15 go little, that it is unnecessary to engrave it. In- 

 CTWC I W *- stead of the stage DL moving, the arm AB containing 

 ^"^ '-"'' the magnifiers slide? up and down. The magnifiers 

 are each set in the centre of a concave silver speculum. 

 This miscroscope was employed by Ellis in his observa- 

 tions on Corallines and the Zoophytes. See Ellis's Et- 

 ta y on the \alitral History of Corallines, and h:- 

 lurnl History of many curious and uncommon Zoophytct. 



VIII. Baric! Microscopes. 



Barrel mi- Single microscopes for opaque olijects are sometimes 

 eroscopts. conveniently fitted up with a barrel or cylinder, on the 

 circumference of which are placed the objects to be exa- 

 mined. 15y turning the barrel round its axis, the dif- 

 ferent objects are brought under the magnifiers, and 

 by a lateral motion of the barrel, other objects may be 

 brought into view. 



IX. On the Magnifying poieer of Single Microscope*. 



O, tb The magnifying power of single microspes increase* 



magnify- with the small neM of their focal length, and may be 

 kif power easily found, by dividing the distance at which the 

 of ><ngl eye tees objects distinctly by the focal length of the 

 lens or globule. This distance varies in different per- 

 ICO|W * tons, and has generally been assumed at 7 or 8 inches. 

 It is obvious, however, that very minute microscopic 

 objects, when examined with an eye of the ordinary focal 

 length, are always viewed at a less distance than 7 or 

 8 inches, in order to obtain the beat possible view of 

 them ; and, therefore, it is this distance that should be 

 ili\ uled by the focal length, in order to obtain the mag- 

 nifying power of the instrument, or the real measure 

 of the help which we derive from it. This distance 

 will be found not to exceed fict inches ; and upon this 

 supposition we have computed the following Table, 

 shewing the magnifying power of single microscope* 

 from riff 10 of * n " K 'h to ' ' ncn - 



Table of Ike Magnifying Puirrr of Small Conte* Lenses, 

 or Single Microscopes, the Distance at tvhiclt the Eye 

 sees dulinclly firing 5 Inchrs. 



Of Ml 



r : 



100 

 75 

 50 



H 



19 

 18 

 17 

 16 

 15 

 It 

 13 

 It 

 II 



<* Tlnm 



V > 

 6.67 



1140 

 I Ml 

 U.M 



?; u 

 *7.7H 

 *9.tl 

 3I.W 



::.*. 1.1 

 35.71 

 '...n 

 4I.6T 

 i - ', , 



MQ 



.-.;. >; 



tuo 



71.43 



100.00 

 IS&OO 

 (MI 



*aoo 



.'.') 



........ 



44 

 100 

 16 



17- 

 <-,.'. 



77* 



s.; '. 

 977 



mi 



1*7-, 

 1481 

 ITM 



*075 



2V) 







510* 



MM 



10000 



277T-' 



MOO 



IfOOOO 



OiM UM rtt of 



IM 



I ' ,T 



MB 



15615 

 I MM 



?U"l 



till. 



1. .'.I- 



MOM 



u,;-. 



:.':r,-, 



MMI 



IMOM 



17111ft 

 211111 



MMM 



I1MM 



1000000 

 19531V 



t'i.".!'7 



156*5000 



1*5000000 



The./ir>*/ column of the Table shews the focal length Compound 

 of the Jens, or globule, in lOOdths of an inch. The ft- micro- 

 cond column shews the number of times that it is mag. ^ope*. 

 nified in diameter, or in one dimension. The third v ^"<"*"^ 

 column shews the number of times that the Mirface is 

 magnified ; and the fourth, the number of times that 

 the cube of an object is magnified. 



As microscopic objects, however, are never mathe- 

 matical lines, and as their solidity, or their magnitude 

 in three dimensions, cannot be rendered visible by a 

 microscope, we consider the tccond column as contain- 

 ing the real magnifying porrtr of the microscope, al- 

 though opticians have hitherto adopted the numbers in 

 the :ir-t column, from an erroneous analogy with the 

 telescope. 



CHAP. II. 

 On Compound Microscopet. 



We have no means of ascertaining, from the descrip- On com- 

 tions of Jansen's microscope, the particular combination pound mi- 

 of lentcs which he employed. It seems more than crotcopti. 

 probable, that the microscopes ustd by him and Gali- 

 leo consisted of a convex object lens, and a concave 

 eye-glass, an.! were nothing more than a short telescope 

 converted into a microscope by lengthening its tube. 



In 1IJ46, Fontana tried two convex lenses; and in all 

 the microscopes which were subsequently used, three 

 or more convex lenses were employed, as in the finest 

 instruments which are now in use. Dr. Hooke, in the i>. 

 Preface to his Micrographia , gives an account of the tluoke'n 

 microscope which he employed. It was about teven micron-op*. 

 inches long, and three inches in diameter, and con- 

 sisted of (our drawing tubes, by which it could be 

 lengthened or shortened at pleasure. It hud ' 

 glasses, viz. a small object-glass, a middle glass, and an 

 eye-glass. When he v. i-Ii, ,1 to have a large field, or to 

 see great part of the object at once, he ustd all the 

 three lenses the middle lense converging upon the 

 eye-glass pencils, which, by their divergency, could not 

 have fallen upon it. Rut when he wished to examine- 

 any individual part with thr greatest distinctness, he 

 removed the middle glass, and used only the other two 

 giaa*e*. 



A description of the microscope of Eustachio Divini, Diini' 

 was hud before the Royal Society in 1668. It consist- microscope. 

 ed of an object glass, a middle-glass, and tiro eye- glasses, 

 which are plano-convex, and placed so as to touch one 

 another in the middle of the convex surfaces. The pur. 

 pose of his construction was to shew the objects flat and 

 not crooked, and to take in a large field at the same 

 time, that it had a high magnifying jxiwcr. It was about 

 Ib'l inches high, and adjusted at four different In 

 In the first, which is the best, it shews lines 41 t 

 larger than they appear to the naked eye ; in tlie se- 

 cond, 90 times ; in the third, 1 1 1 times ; and in the 

 fourth. 143 times. The diameter of the field, or 

 subtense of the visual angle, measured upon the ob- 

 ject plate, in the Jint length, was 8 inches 7 lines ; in 

 the second, 12 inches 4 lines; in the third, 13 inc.: 

 and in the fourth, a little more than 16 inches. The 

 tube was as large as a man's leg, and the eye-classes 

 like the palm of the band. See Phil. Tram. 1668, No, 

 42, p. 8 12. 



Philip Konnani, in a work entitled Obseroationes cir- Ronnani's 

 ca I'irentia, qua in rebut non vivenliltus rr/.erinntitr, $c. microtcope. 

 l6yi, published an account of two compound micro. 1691> 



