296 RAMBLES OF A NATURALIST. 



apparatus, M. Oberhaiiser devised his microscope pan- 

 cratique, while the Nachets added several ingenious ac- 

 cessories to their special movement. The impulse which 

 had been given to this branch of scientific invention 

 originated in France, but was soon followed in other 

 countries, and it may be said that the possible limits 

 which micrographers hoped to attain thirty years ago 

 have long since been exceeded. 



The qualities which a good microscope ought to possess 

 are to give the image with distinctness. The observer 

 ought to be able to see as well in his instrument as 

 he sees on the table at the distance of distinct vision. 

 Our skilful workmen of the present day can fulfil these 

 requirements until they reach a magnification of 400 or 

 450 diameters.* Beyond that limit, the outlines become 

 less clear, and the images grow more and more confused. 

 However, it is sometimes of use to go higher than this, 

 and we may even, if we wish it, exceed a thousand 



after having held the chair of Geology in the Faculty of Sciences at 

 Toulouse. This fact is in itself sufficient to indicate a variety of 

 acquisitions, whose extent can only be appreciated by those who 

 are personally acquainted with him. M. Dujardin has especially 

 devoted his attention to the lower organisms, and, in this respect, he 

 has enriched science with many extremely important discoveries. 

 "NVe may instance, amongst others, the fundamental fact which he 

 discovered in reference to the Rhizopoda, which, according to this 

 observer, can fabricate for themselves, as it were, the members 

 which may be momentarily necessary to them, this being effected 

 by means of the homogeneous substance of their body. Indepen- 

 dently of a very great number of special memoirs, which for the 

 most part have appeared in the Annales des Sciences Naturelles, 

 M. Dujardin has published, in the Suites a Buffon, works on the 

 Infusoria and on Intestinal Worms. 



* The magnifying power of a microscope or lens is estimated in 

 diameters. To obtain the magnification of the entire surface, we 

 must square the given number ; thus, a magnifying power of 450 

 diameters will in reality enlarge the object 202,500 times. ^ 



