LIGHT (OPTICAL) MICROSCOPY 



lens. With the flame of the lamp, the glass 

 ball and the lens coaxially lined up he at 

 first got enough light, "but after a certain 

 degree of magnifying they leave us again 

 to the lurch." 



In one of his models Hooke used two 

 lenses and filled the space between them with 

 water. Such an arrangement was an improve- 

 ment of Descartes' water tube described 

 about 30 3^ears before. Hooke praised the 

 brightness of the image obtained in this way, 

 but he did not use it much "for other in- 

 conveniences." The pictures in Hooke's book 

 are considered by some historians the first 

 precise and detailed technical observations 

 depicted which we possess. 



Another important microscopist of this 

 time was Johannes Hevelius, (lGIl-1687) a 

 German astronomer who devoted much time 

 to the study of other scientific fields. His 

 Machina Coelestis was printed in two vol- 

 umes, the first of which, published in 1673, 

 contains descriptions of his microscopes. He 

 expressed his admiration for the English 

 microscopes, probably of Hooke's design, 

 but improved it by his invention of a micro- 

 metric fine focusing device. It consisted in a 

 vertical screw-threaded rod, rotating in a 

 spherical nut which pushed a sliding sleeve 

 along another rod upon turning of the screw. 

 The body of the microscope was fastened to 

 the sleeve Avhich, upon traveling along the 

 rod, would gently lift or lower the micro- 

 scope tube. With this "fine adjustment" 

 microscope design made another important 

 advance. 



In the following century further progress 

 in design and applications of the microscope 

 was made with substantial participation of 

 amateur microscopists, among whom Antonj 

 van Leeuwenhoek must be mentioned as the 

 most famous and successful. He lived all his 

 life from 1632-1723 in Delft, Holland, where 

 he was active in business on a small scale as 

 a haberdasher, draper. Chamberlain to the 

 Sheriffs, and surveyor. He was an amateur 

 without any scientific education and special- 



ized in optical lens grinding and polishing, 

 in which art he acquired a high professional 

 skill. All his observations and discoveries 

 were made with homemade single lens mi- 

 croscopes, but no description of the manner 

 of manufacturing or of using them was left 

 by Leeuwenhoek. He never divulged his 

 secret methods with which he could out- 

 perform all other microscopists for at least a 

 century. 



However, a few of his original microscopes 

 are still in existence and have been studied. 

 They have all very small double-convex 

 lenses mounted in a socket between two 

 metal plates, about 1.5 X 3.5 centimeters, 

 riveted together. The object was held by a 

 needle near the socket by means of which 

 the object could be brought into the proper 

 position. The lenses had a focal length of 

 from fractions of a millimeter to about five 

 millimeters, and a magnifying power from 

 about 50 times to 270 times. Coarse and fine 

 focusing were accomplished by means of 

 two screws, one longer and vertical and 

 another shorter and horizontal, the latter 

 being fastened crosswise on the former by a 

 nut piece which held the object needle. Upon 

 proper operation of these two screws, the 

 object could be accurately focused and 

 rotated. 



Twenty-six of Leeuwenhoek's microscopes 

 were bequeathed to the Royal Society, where 

 they all vanished about a century ago. The 

 rest Leeuwenhoek left with his daughter 

 when he died. After her death in 1745, they 

 were auctioned off and scattered; most of 

 them were acquired by Dutchmen. From 

 the sales-catalog Leeuwenhoek seems to 

 have left 247 complete microscopes with 

 lenses and objects still in place and in addi- 

 tion to these 172 lenses mounted in their 

 plates. Three lenses were made from quartz. 

 The plates were mostly made from silver 

 but three were of gold and sold by weight. 



His observations and discoveries Leeu- 

 wenhoek described in letters (165 of them) 

 mostly directed to the Royal Society in 



458 



