J. W. GORDON ON A " NEW " OBJECT GLASS BY ZEISS. 517 



of the specimen. A shallow brass ring enables the observer 

 to move it about, and place it wherever he pleases on the 

 specimen, so that it exactly covers the spot which he desires 

 to examine. If, then, he places this specimen with this sup- 

 plemental lens in position under his dry objective, he gets, 

 in effect, precisely the combination which Mr. Nelson describes. 

 It is to be observed that in this combination it is not necessary 

 to make any corrections for colour or for spherical aberration, 

 because if the lens is of the right thickness, so that the centre 

 of its spherical surface coincides with the focal point, then the 

 incident beam passes the air-glass surface of the lens without 

 refraction. It passes, therefore, without aberration of any kind, 

 and the dry lens is in exactly the same position as if it were 

 applied to a dry object. In the case of a dry lens which is 

 corrected for the cover glass this would, of course, be a dis- 



z 



COVER CLASS. 



BRASS RING. 



-LENS. "--GLASS BASE. 



Fig. 1. 



advantage, but a dry lens which is adapted to be used upon 

 a dry object will give, under these conditions, a perfect image. 

 That is, no doubt, the principle of construction of the Zeiss lens 

 which Mr. Nelson describes. 



To complete the description of this new adjunct : The brass 

 ring is mounted upon a thin cover glass which, in its turn, 

 carries the spherical lens cemented at its centre. Theoretically, 

 of course, the lens and cover glass should be of the same glass, 

 but in the case of my model I have used what I had ready to 

 hand, without being punctilious upon this point. 



Such a supplemental lens is for some purposes a very con- 

 venient adjunct to the ordinary microscope. In the first place, 

 it puts the microscopist in possession of a system such as Mr. 

 Nelson has described, at extremely small cost, for, of course, this 

 appliance, consisting simply of a small brass ring, a cover glass, 

 and an uncorrected spherical lens cemented together, can be 

 produced at almost infinitesimal cost. It is then available for 



