198 THE USE OF THE MJC'BOSCOPE 



lenses in front. One early form had a triplet in front, 

 and a doul)let in the middle, while the back lens was again 

 a triplet. The use of eight components, of course, added 

 to the cost. Much of the adjustment in such an objective 

 was done by trial of different distances, and tentative 

 combinations were made of components slightly differing 

 in their curves, until the test objects were best seen. 

 Thus, there was no uniformity in the objectives up to 

 the time of Abbe's method of previous complete calculation, 

 and exact workmanship, with accurate tests of the curves. 

 However, Ross, in 1837, invented the correction for different 

 thicknesses of cover-glass by moving the front lens farther 

 from (for thinner covers), or nearer to (for thicker covers), 

 the other lenses. Wenham, in 1855, improved this some- 

 what by moving only the back lenses, the front lens or 

 lenses being stationary. Wenham, in 1850, also invented 

 the single small uncorrected front lens to the objective, 

 serving mainly as the magnifier, the corrections being 

 taken by the two back doublets; and this invention facili- 

 tated the manufacture of objectives. 



The Water-immersion Objective. — The making of water- 

 immersion objectives, on this model, was carried out in 

 the 'sixties and 'seventies, especially by Hartnack, reaching 

 its zenith about 1877. Some of these water immersions 

 had an aperture of 1.23, and many discoveries were made 

 by them. A look at Carpenter's classic book on the 

 microscope, in its 1875 edition, will show the state of micro- 

 scopical invention at and before that date. In England, 

 achromatic condensers were often employed; but, on the 

 Continent, the use of the concave mirror, with its small 

 cone of light of about 0.3, and its considerable aberration, 

 due to obliquity, was universal. 



Abbe's Inventions. — Abbe, in 1873, had given an account 

 of his wide-angled condenser, used preferably as a water 

 immersion. It was not corrected, since only a cone 

 of about 0.3 was required; and this could be delivered at 

 various angles, thus replacing the concave mirror. In 

 1873, Abbe pubhshed his theory of microscopical vision, 



