516 J. W. GORDON ON A "NEW OBJECT GLASS BY ZEISS. 



centre of the sphere lies in the object. The spherical surface, 

 therefore, produces no refraction, and its addition to the optical 

 system involves no change in the correction of an objective 

 adjusted for viewing an uncovered object. The abolition of the 

 top surface of the cover glass by oiling on the supplementary 

 front lens produces an increase of 50 per cent, in magnifying 

 power, and a commensurate increase in light-gathering power. 

 The catoptric haze produced by internal reflection from the front 

 face of the permanent front lens sinks into comparative in- 

 significance, and a g-in. dry lens is converted into a 3 -in. 

 immersion system of much improved defining power." 

 Two things may be added to this description : 



(1) The numerical aperture of this lens as it stands is 0*54. 



(2) It was intended that this front lens should be made 

 adaptable not only to the g-in. mentioned, but also to my |-in. 

 objective, in which case it would have yielded exactly the 

 combination which Mr. Nelson now describes. It was found, 

 however, that the front lens was a little too thick for use with 

 a I -in. objective, and consequently I have never been able to 

 adapt it to a higher power than the g-in. The principle, how- 

 ever, upon which the construction is based is clearly set out 

 in the extract above given from the catalogue of the Optical 

 Convention, and if Messrs. Zeiss have given any attention to 

 that document, it is obvious that for upwards of a twelve-month 

 past they have had the benefit of the suggestion so made public. 

 It is, of course, quite possible that the Jena House have paid no 

 attention to the catalogue of the Optical Convention, but have 

 worked out the theory of this new objective for themselves. 

 Even if we suppose that they have profited by the publication 

 which has been placed at their service, we must still concede 

 to them the merit of being the first to turn to account a sug- 

 gestion which has been equally at the disposal of our own 

 British manufacturers. 



While I am on this point I should like to communicate to 

 the members of the Club a further development of this principle. 

 In consequence of the failure of this lens, as I have explained, 

 to serve the purpose as an oil-immersion front lens for my |-in. 

 objective, I was led to provide myself with another, of which 

 a sketch appears as fig. 1. It is adapted, as will be seen, to 

 be mounted, not on the dry objective, but on the cover glass 



