LIGHT (OPTICAL) MICROSCOPY 



in 1755. The latter was producing lenses The method of finding the best combina- 



with reduced chromatic aberrations on a tions of lenses for the desired effect was 



commercial scale from that time on. more or less trial and error, leading to a 



The decisive experiments had been made tremendous waste, and it was Carl Zeiss in 



by Hall, who found that flint glass differs Jena, Germany, who was no longer willing 



only slightly in refractive power from or- to accept this situation. In order to bring 



dinary crown glass, though its dispersion is to an end "das ewige bei uns Optikern ge- 



more than twice as large. This led to a co- brauchliche Probieren" meaning "the eternal 



axial combination of a strong positive crown trying out of lenses so customary with us 



glass lens with a weaker negative lens of flint opticians," Zeiss formed a partnership with 



glass, which compensates the chromatic Ernst Abbe, a young physicist at the Uni- 



aberration of the other lens but reduces its versity of Jena, who cleared the theoretical 



magnification to only about one-half. This background and calculated better and better 



is the type of lens which Dollond brought lenses in the future. From 1883 on apochro- 



on the market for telescopes from about 1760 matic objectives were made which were 



on. The difficulty in making perfect chro- corrected both for achromasy and for spher- 



matically corrected lenses for the microscope ical aberration at three different wave 



seemed unsurmountable because of their lengths. Because glass with new optical 



necessary exceedingly small size. properties was needed, Schott's Optical 



Onlv very few experimenters were success- Glass Works were founded at Jena. It pro- 



ful in their efforts at that time. An exam- duced all the optical glass needed by Zeiss. 



pie is an achromatic microscope objective This cooperation turned out to be so ad- 



in the possession of the Utrecht Uni- vantageous that the Dutch and English 



versity Museum which had been made monopoly in optics was no longer unchal- 



by an amateur in 1791. He was a lenged after the second half of the 19th 



eavahy colonel, named F. Beeldsnijder. century. 



Even professional opticians, such as Fraun- An important contribution to the optical 



hofer in Munich, Amici in Modena, Charles art was made by Abbe in 1878 by the design 



in Paris, and others, were failing in the first of immersion objectives, the principles of 



quarter of the 19th century. However, as which had been well known for about 200 



early as 1807, Harmanus van Deijl, optician years. In the 17th century Hooke had made 



in Amsterdam, Holland, had produced satis- the observation that he got much clearer 



factory achromatic objectives for sale and pictures of the "animalcules" living in water 



had published his efforts and results. Ob- when the front lens of his microscope touched 



viously, it had been impossible for a long the water so that there was no air between 



time to learn from his experiences. the lens and the object. The explanation is 



In 1824 a new principle for making higher- that the resolving power of an objective is 



powered achromatic objectives for the mi- proportional to its aperture or to the vertex 



croscope was introduced by the French angle of the cone of light which the front 



physicist Selligue, who suggested screwing lens admits. This aperture is much reduced 



several low-powered achromatic lenses to- by total reflection of the light coming from 



gether. In this way, a higher magnification the object and going in one case, either 



was obtained without grinding and polishing through water and air or, in the other case, 



lenses with very short focal length. This through water, a cover glass, and air into 



principle was used and improved by the the front lens. 



commercial opticians of that time. Chevalier In Hooke's experiments this loss Avas 



in France, Amici in Italy, and Lister in avoided because the interspace between the 



England. object and the front lens of his microscope 



460 



