IMMERSION OBJECT-GLA.SSES FOR THE AIICROSCOPE. Ill 



that as yet it has not been recorded that the enamel of the 

 elephant's molar, as also that of the mastodon's, presents the 

 very same decussating arrangement of the inner portion of 

 its enamel which Mr. Tomes has figured ('Phil. Trans.,' 

 1850, pi. xliv, xlv, xlvi), as noted by him in the Rodentia, 

 less the Leporidse and Hystricidte. Thus the rodent affinities 

 of the Proboscidea, which have so often been commented 

 upon, receive a fresh illustration. 



Note on Immersion Object-glasses /or the Microscope. 

 By Dr. Royston-Pigott, M.A., F.C.P.S., F.R.A.S. 



It is a curious fact that, though several writers in this 

 country and America have taken up the question of immersion 

 objectives, very different opinions have been expressed as to 

 the causes of the advantages obtained. One writer boldly 

 asserts that the aperture of an immersion lens can never 

 exceed 80°. The following experiment points to a very 

 different conclusion. 



A vessel of glass is filled with water up to the brim ; a 

 brilliant paraffin flame is placed at about four feet from it, 

 with its centre at about half an inch below the level of the 

 surface of the water. A lens capable of transmitting parallel 

 rays is then so placed that its focus coincides with the centre 

 of the flame ; a beam of parallel rays passes through the plane 

 side of the glass vessel Avithout deviation, in a horizontal 

 direction. The body of a microscope, armed with the im- 

 mersion object-glass, is then so inclined with the immersion 

 front dipping into the water that the light may just become 

 visible on looking down the tube, The inclination of the 

 axis of the tube or body to the horizon gives the semi-aper- 

 ture. This was found to be fifty-three degrees and a half. 

 The angular aperture of the pencil capable of entering from 

 water into this immersion lens was, therefore, not 80°, but 

 107°. This clearly depends upon the refractive index of the 

 front glass of the objective, and upon the refractive index of 

 water. This verbal description will be rather more per- 

 spicuous by reference to a diagram; l is the lamp ; c, the 

 condenser, transmitting parallel rays, without deviation, 

 through the flat side of the vessel of water (w) to the im- 

 mersed object-glass screwed into the body of the microscope 

 (m), whose semi-aperture was thus found to measure 53° 30'. 



But it should be stated that immersion lenses differ cou- 



