216 



MICROSCOPE. 



friend Cornelius Drebell, the very same microscope 

 which Zacharias Jansen and his father had presented 

 to Prince Maurice, and Albert, archduke of Austria. 

 This instrument was six feet long, consisting of a tube 

 of gilt copper, an inch in diameter, supported by thin 

 brass pillars in the shape of dolphins, on a. base of ebony, 

 for the purpose of holding the objects to be examined. 

 There is reason to think, that this was nothing more 

 than a telescope converted into a compound micro- 

 scope. Cornelius Drebell, therefore, who has common- 

 ly been considered as the inventor of the microscope, 

 appears to have derived this honour from the acciden- 

 tal circumstance of his having exhibited the micro- 

 scope made by Jansen ; and as he was a favourite at 

 the court of James VI. where he lived some time, this 

 opinion may have proceeded, not only from his own 

 arrogance, but from the influence of royal favour. 



Viviani expressly informs us, in his life of Galileo, 

 that this great man was led to the construction of the 

 microscope from that of the telescope ; and that, in the 

 year 1612, he actually sent a microscope to Sigismund, 

 King of Poland. Dissatisfied with the performance of 

 this instrument, he appears, from his letters, to have 

 been much occupied about 1624 in bringing it to per- 

 fection, but we have no information respecting the re- 

 sult of his labours. 



As there is no reason to believe that the microscopes 

 invented by Jansen consisted of two convex lenses, 

 like those new in use, the honour of this improvement 

 seems to be due to M. Fontana, a Neapolitan, who first 

 described it in his work entitled Nonce Terresirium ft Ce- 

 letlium Observationes, Neap. 1646. Fontana distinctly 

 assumes the merit of this improvement, and as no other 

 person has laid claim^to it, we agree with Montuda in 

 allowing him the possession of this honour *. 



In pursuing this subject, we shall confine ourselves 

 solely to the description of microscopes, supposing that 

 the reader is acquainted with the principles of their con- 

 struction, which fall to be explained under the article 

 OPTICS. We shall treat, 1st, On Single Microscopes; 

 2dly, On Compound Microscopes ; Sdly, On the Solar 

 Microscope ; 4thly, On the Lucernal Microscope ; and, 

 5thly, On Microscopic Objects. 



CHAP. I. 



O* Single Microscope*. 



On single A Single Microscope, is an instrument in which only 



micros- a single lens or mirror is employed for the purpose of 



copes, magnifying objects. If the instrument is furnished 



with mirrors or lenses for illuminating the object, or 



with any other apparatus, however complicated., it still 



conies under the denomination of a single microscope. 



Single mi- One of the simplest of all single microscopes, is a 



croscope plano-convex, or a double convex lens of glass, fitted 



with lenses into a rim of brass furnished with a handle. The ob- 



of glass. j ec t ; g then held in the anterior focus of this lens, and 



appears magnified in proportion to the smallness of its 



focal length. 



r.cewcn- Microscopes of this kind were first successfully ap- 

 hoek's mi- plied to the examination of natural objects by the cele- 

 troscopc?. brated Anthony Van Leewenhoek. They consisted of 

 a small double convex lens, carefully ground and po- 

 lished, and inserted between two thin plates of metal, 

 pierced with a hole smaller than the diameter of the 

 Jens, and afterwards rivetted. The object was placed 



upon a silver point or needle, which, by the agency of 

 screws, could be turned in all directions, ami placed at 

 any requisite distance from the lens. To this needle 

 the objects were fixed with glue when they were solid ; 

 and when they were fluid, he put them on a thin film 

 of mica, or blown glass, which was afterwards attached 

 to the needle by glue. In all Leewenhoek's micro- 

 scopes the lenses had not a very high magnifying power; 

 and there is reason to believe, that most of his discove- 

 ries were made more from the distinctness arising from 

 the accurate figure and good polish of his lenses, than 

 from the greatness of their power. Of the 26 micros- 

 copes which he presented to the Royal Society, only 

 one had a focal length so small as Vs-t' 1 f an inch, and 

 all the rest were below lialf an inch in focal length. 



Dr. Hooke appears to have been the first who sub- 

 stituted glass globules in place of convex lenses. In 

 the preface to his Micrographia, published in 1665, he 

 describes the method of making them. A small rod 

 or strip of thin and good window glass is drawn out 

 into fine threads, either by holding it in the flame 

 of a candle, or a lamp with spirit of wine or good 

 oil. The end of one of these threads is then held in 

 the flame till it runs into a small drop or globule of the 

 required size. When the globule has cooled, it is fixed 

 upon a thin plate of brass or silver, so that the centre 

 of it is directly over the centre of a very small hole in 

 the metallic plate. In this way, none of the rays of light 

 that issue from an object placed in its focus, pass through 

 the part of the globule where it is joined to the glass 

 thread. Dr. Hooke also fixed the globules with sealing 

 wax to the end of a stick, so that the threads stood up- 

 wards, and he ground off" the ends of the threads, and 

 polished them. 



The art of making microscopic globules of glass, was 

 brought to great perfection by Father di Torre of Na- 

 ples. Having formed glass globules by the method 

 already described, he separated the ball of glass from 

 the thread by the sharp edge of a piece of flint. When 

 a great number of globules were made in this manner, 

 he next proceeded to give them a complete spherical 

 form, by melting them a second time. For this pur- 

 pose, he made use of a piece of tripoli about four or 

 five inches long, and three or four inches thick. After 

 it had been calcined, by surrounding it with charcoal 

 nearly red hot, and allowed to grow quite cold, several 

 small hemispherical cavities of different diameters are 

 made on the flat side of it, and are nicely polished and 

 neatly rounded at the edge. The small glass globules, 

 after being carefully cleaned, are placed in the cavities 

 of the tripoli by a pair of delicate nippers. The ex- 

 tremity of the flame of a blow-pipe is then directed to- 

 wards the globules, which assume a perfectly spherical 

 form when brought to a fluid state, and never adhere 

 to the tripoli. When the globule is cold, it is cleaned 

 by rubbing it between two pieces of paper, and is then 

 set in a brass cap for the purpose of trying its figure, 

 by viewing an object through it. It sometimes hap- 

 pened, that in damp weather only/or or Jive globules 

 out of forty were fit for use. In the year 1761, four of 

 Father di Torre's microscopes were sent in a present to 

 the Royal Society. One of them had a diameter of 

 jJgth of an inch, two of them a diameter of T ' T th part of 

 an inch, and the fourth a diameter of only the ri^th of 

 an inch. Mr. Baker, who examined these globules 

 with great care, could not make use of the smallest of 

 them, and considered them as more curious tlian useful. 



Single Mi- 

 croscopes. 



Hooke 1 * 



microscopic 



globules. 



Method of 



making 



them. 



Di Torre's 



micros- 

 copes. 



Dr. Priestley has completely miranderstood Monrucla's observations on this subject, when he represents him as ascribing to Fontans 

 the invention of compound microscopes. See Montucla's Hat. dtt Math. torn. ii. p. 239 ; and Priestley On Vision, vol. i. p. 78. 





