J. W. GORDON ON A " NEW " OBJECT GLASS BY ZEISS. 519 



its upper surface dry and through the mounting medium and 

 the specimen. The specimen is represented by two opaque 

 objects, one in optical contact with the under surface of the 

 cover glass, the other at some little distance below the surface, 

 so that there is no optical contact in the second case. A line 

 marked R indicates how light from the condenser is reflected 

 downward from the upper surface of the cover glass, and then 

 upward from its lower surface so as to produce a luminous haze 

 overspreading the field except where optical contact between the 

 specimen and the cover glass does away with the second reflection. 

 It will be seen that the shadows produced by transmitted light 

 are attenuated by this reflected light wherever the object does 

 not come into optical contact with the cover glass. Similar 

 considerations show that the light which is internally reflected 

 from the upper surface of the front lens and again internally 

 reflected from the lower surface of that front lens gives rise to 

 a similar catoptric haze which is diffused over the whole field and 

 serves therefore to attenuate the absorption image even of these 

 objects which are in optical contact with the cover glass. 



Now all this mischief, so far as it is due to internal reflections 

 in the cover glass, is avoided by the use of a spherical lens 

 cemented on to the cover glass, since by its means all such 

 internal reflection is abolished. From this cause chiefly results 

 the improved definition which gives to these immersion objectives 

 of low angle their comparatively high resolving power. 



Besides its extreme cheapness, this new form of immersion 

 lens has the merit of serving as a finder. For example, an 

 observer who wishes to keep a particular object say a culture 

 under observation for a length of time can cement one of these 

 supplementary lenses in place over the spot occupied by his 

 specimen and put the specimen away in that condition. When 

 next he goes to examine it he will find it without the least 

 difficulty, for all that he has to do is to place his slide on the 

 stage of his instrument with this cemented lens in the axis of 

 collimation. It is possible, now that Messrs. Zeiss have dis- 

 covered and advertised the value of low-power immersion objec- 

 tives, that this very simple appliance may also find a manufacturer. 

 Hitherto, I have not succeeded in interesting any of our 

 manufacturers in it, possibly because the market price would 

 necessarily be small. As against that consideration, however, 



