GLASGOW SOCIETY OF FIELD NATURALISTS. 121 



Barlow, and Mr. Coddington ; he joined a plano-concave flint 

 lens and a convex crown glass lens together by means of the 

 transparent medium Canada balsam, which may be taken as the 

 basis of onr present excellent object-glasses, as by the cementing 

 of the lenses together nearly half of the loss of light from 

 reflection is prevented, which is very considerable in consequence 

 of the numerous surfaces in a compound achromatic object-glass. 

 Since that time the late Mr. A. Ross, in London, was constantly 

 employed in bringing object-glasses to greater perfection by 

 applying Mr. Lister's principle of cementing the lenses together 

 with Canada balsam, and he succeeded in balancing the errors of 

 chromatic and spheric aberration so well that the circumstance of 

 covering the object with the thinnest glass disturbed its correction. 

 This unsuspected difiiculty was finally overcome in 1837, when 

 Mr. Eoss was able to announce to the Society of Arts that he had 

 succeeded in doing so by making the distance of the anterior 

 lenses of the object-glass to the middle combination adjustable by 

 means of a screw-collar. Other makers followed soon, among 

 whom notably Smith and Beck, Powell and Lealand in London ; 

 Hartnack, the successor of Oberhauser, and Nachet in Paris ; 

 Gundlach in Bei-lin ; Zeiss in Jena ; Schick, Merz ; and in America 

 P. Tolles and Zentmayer. The latest and greatest improvements 

 in object-glasses have been efiected by Mr. P. H. Wenham, in 

 1873, who reduced the number of surfaces by six, thiis securing a 

 great increase of brilliancy and definition. 



At the beginning of the present century the microscope had 

 fallen rather into discredit, and began to be neglected, but since the 

 introduction of achromatic object-glasses it has been improved, not 

 by steps, but by leaps and bounds. I must not forget to mention 

 that prince of microscopists, the great Professor Ehrenberg, of 

 Berlin, who astonished the world by his wonderful researches and 

 discoveries, and who was principally instrumental in restoring the 

 credit to the microscope, which it seemed to have lost for a while. 

 In 1846, Mohl, an eminent German savant and microscopist, 

 stated that the limit of the best microscope at that time was a mag- 

 nification of 300 to 400 diameters, or 90,000 to 160,000 times, 

 superficially, and that any further magnification would only result 

 in loss of light and definition. In 1840, however, Powell and 

 Lealand made their first celebrated object-glass of one-sixteenth of 

 an inch focus, magnifying nearly 1000 diameters, and in 1860 they 



