Immersion Lenses and New Befractometers. 71 



from the object, necessarily effected by the use of dry objectives with 

 a Canada-balsam mounted object. 



I believe that in this country the first notice of immersion lenses 

 was by Mr. Mayall, jun., in a paper, Feb., 1868. Comparing the 

 two methods, he says : — " The relative separating and defining 

 power of the two systems have not received that attention which it 

 deserves." 



Mr. Mayall continues : — " It is not difficult to see that Amici's 

 system of connecting the objective with the cover-glass by a film of 

 water very much diminishes the reflexion which necessarily takes 

 place in the incidence of the oblique hght when the diy objective 

 is employed. The limiting angle of refraction in water being about 

 48 degrees (sic), it follows that whatever is the degree of obHquity 

 in the incident light on the object, the immersion objective never 

 has to do with rays of greater obliquity than 48^. To this, in 

 great measure, is due the greater clearness and precision of image 

 obtained. Continental opticians and men of science have been 

 aware of the merits of the immersion system during several years 

 past ; and to such purpose, that knowing how little attention it has 

 received here, they do not scruple to say that the English no longer 

 take the lead either as opticians or as microscopists." 



This glaring error of 48° most iwohdbly arose from the uriter 

 taking the angle of total internal reflexion hettceeu ivater and air 

 instead of ivater and glass. 



In the ' Popular Science Eeview ' (p. 330, 1868), the talented 

 Editor remarks : — " Hartnack's immersion lenses appear to be get- 

 ting into general use for work which does not require such high 

 powers as the -^Vth to aVth objectives ; but the subject of immersion 

 lenses requires to be more fully worked out than it has been. We 

 want experience of those who have employed both forms of object- 

 glasses, and who can fairly and fully state the respective quahties of 

 each." 



In the ' Monthly Microscopical Journal,' Dr. Lawson observes 

 (p. 217, vol. ii.) :— 



" Mr. Ross's New Immersion Lenses. — Mr. Eoss has just pre- 

 pared a number of immersion lenses, which our readers will do 

 well to examine. There is a decided improvement over the dry 

 glass in definition, and there is vastly more hght." 



A recent writer in this Journal (Mr. Wenham) seems to follow 

 in the wake of Mr. Mayall, for he states (p. 17, XXV.) :— " The 

 same optical law that limits the aperture of any object-glass to 

 near 82° in a balsam-mounted object also determines the angle in 

 the lens at which the rays diverge after being refracted from the 

 plane surface of the front. This can never exceed 82° in a dry 

 objective, nor can it be greater on the immersion system." Verhum sat. 



