394 Transactions of the Society. 



XXV. — Note on Homogeneous Immersion Ohject-gh 

 By F. H. Wenham, F.R.M.S. 



iRead Uth May, 1879.) 



As the homogeneous immersion object-glass is revived as a recent 

 idea, I quote from a paper contributed by me to the ' Monthly 

 Microscopical Journal,' June 1st, 1870, the following comment in 

 favour of homogeneous immersion. " One advantage in the immer- 

 sion objective is, that it almost prevents the loss of light from the 

 reflection of the upper surface of the cover and front of lens, and in 

 part neutralizes any error of figure and polish that may exist be- 

 tween them. There is also another condition annexed ; it has the 

 singular property of a front lens of adjustable thickness, and there- 

 fore can be set to the utmost nicety to balance the aberrations. Of 

 course there is no optical advantage attendant upon the use of water. 

 If a medium of the same refractive power as the glass were to be 

 employed, the result would be better. Water having a low refrac- 

 tive index, an adjustment is required for each thickness of cover, 

 and a difference of adjustment is not so marked and sensitive as in 

 the ordinary dry objectives ; but if a medium of similar refraction to 

 the glass were to be used, no adjustment would be required for any 

 thickness of cover, supposing the test objects to be mounted thereon 

 (which they generally are) ; for, in fact, we should then view them 

 all with a front of the same thickness — considering the cover, the 

 front lens, and the interposing medium as one." * 



[Since the above was read, Mr. Wenham has sent U3 th e 

 following additional note on this subject. — Ed. ' J.E.M.S.'] 



In the ' Quarterly Journal of Microscopical Science,' No. 11, 

 1855, p. 3!J3, I described as part of the system of an object-glass, 

 a front lens connected with the slide by an intermedium of Canada 

 balsam ; of this I say, " It will be seen from the position of the 

 object, that each ray of light passing from that point through the 

 surface of the hemisphere, will be transmitted in straight lines in 

 a radial direction without undergoing any refraction; the conse- 

 quence of which is, that the full and undiminished aperture of the 

 object-glass is made to bear upon the object." Of the actual per- 

 formance obtained by this object-glass (a i), I say on the next page : 

 *' When an object is seen under these circumstances, it at once 

 shows the great increase of distinctness that is to be obtained in the 

 structure of the more difficult Diatomaceous tests when they are 

 thus viewed in Canada balsam with the full aperture of the object- 

 glass. Markings which in the neighbouring dry objects of the same 

 character are scarcely discernible, are sharply and distinctly visible 

 under the balsam hemisphere, with the same illumination." 

 * The discussion on this Note will be found at p. 494. 



