WITH THE MICROSCOPE. 285 



of the tube of the microscope being increased considerably in 

 length. By a working glass, I mean one that can be employed with- 

 out great trouble or difficulty, and does not require any elaborate 

 arrangements with regard to illumination, adjustment, &c. In fact, 

 it works fairly even without a condenser of any kind, the common 

 concave mirror being alone used. There is plenty of room for 

 focussing, although, of course, specially thin glass or mica must be 

 employed. I have made and published many drawings of tissues 

 of the higher animals magnified with this glass, and it need scarcely 

 be said that, as it can be brought to bear upon textures of this class 

 (even bone and teeth), thin sections of which are obtained only with 

 great difficulty, it must be readily applicable to other departments of 

 microscopical enquiry. Object-glasses of very high magnifying power 

 have been more recently made by other makers. Hartnack's high 

 power immersion objectives are among the best on the continent. 

 An objective of high magnifying power (a twentieth) with a single 

 front lens was made three or four years ago by Messrs. Smith and 

 Beck. The magnifying power is about one third less than that 

 of the twenty-fifth, and it appeared to me that the definition was 

 not so good. The amount of light admitted was ample. It is, 

 however, exceedingly difficult to express in words the merits of one 

 glass as compared with another, and there can be no doubt that an 

 observer who has used one glass very much, especially if he has 

 made new observations by its aid, is almost of necessity prejudiced 

 in its favour. I confess that, unless I had worked with an objec- 

 tive for a considerable period of time, I could not express a 

 decided opinion as to its qualities. The difference between the 

 working powers of the glasses of the best makers is, at most, 

 very slight, and not to be demonstrated without the most exact and 

 careful examination. At the same time, it is certain that the slightest 

 advantage in defining power ought not to be underrated, for it may 

 enable the observer to see some scarcely perceptible, but neverthe- 

 less most important, points not observed before, and in some 

 instances the very slightest advantage of this kind may necessitate a 

 complete alteration in general views up to that time received as true, 

 and even considered to be fixed and unalterable. Improvement in 

 the means of observation is of the utmost importance, and, however 

 slight, is almost invariably soon followed by the discovery of new facts. 

 357. " immersion " Twenty-fifth. - - As already stated in p. 6, 

 Hartnack, of Paris, has made some excellent lenses of high magni- 

 fying power upon the " immersion " principle. Messrs. Powell and 

 Lealand have recently made a twenty-fifth object-glass upon the 

 same plan. It possesses the following advantages : 



