485 



delicate test for the truth of lens surfaces, for, roughly speaking, 

 the colours run through the complete range of the spectrum for 

 every increase of the space between the adjoining surfaces of the 

 lens and proof-plate by -q^qo^ part of an inch, and it is quite 

 easy to detect irregularities in the surfaces of so small an amount 

 as the 100 ^ 000 th part of an inch. When a lens is absolutely 

 correct to the proof-plate, the appearance is that of one uniform 

 tint of colour over the entire surface. In the final stages the 

 lens is once more mounted on the lathe-spindle, to be centred. 

 The various constituents are then cemented together and baked 

 for several hours, are subsequently mounted in their brass 

 fittings, and the adjustment for axial truth and distance completes 

 the process." 



In acknowledging a very hearty vote of thanks for the 

 demonstration, Mr. Watson Baker said some people seemed to 

 think that there was some occult influence in Jena glass. People 

 would ask whether a given objective contained or was made of 

 Schott or Jena glass, and, on being told that such was the case, 

 would go away quite satisfied. Other people have the idea that 

 any glass may be used, so long as it is properly ground and 

 polished, and have no knowledge at all of the properties of Jena 

 glass. In a catalogue issued in 1180 by an eminent firm of 

 optical-glass makers only six kinds of glass are listed, and from 

 these the optician's work had to be done. Then came the Jena 

 glasses with their enormous possibilities. The Schott works now 

 catalogued eighty varieties as regularly stocked, and of these 

 varieties there are many variants — sometimes as many as a 

 hundred of one kind. It is only by having at command 

 such a variety of glasses that the modern objective has been 

 made possible. Referring in more detail to the exhibits before 

 the meeting, the speaker said that the old method of testing 

 lenses in process of grinding was by using a templet of metal cut 

 to the required curve, and it was left to the worker to decide 

 whether or not the lens fitted. A modern objective represents 

 the genius of the mathematician, the skill of the optical-glass 

 maker, and of the mechanician and supervisor, all combined to 

 produce the effect obtained. 



Among the presentations made to the Club was a copy 

 (presented by Dr. Spitta, President of the Club 1904-7) of the 

 second edition of his valuable work on " Microscopy." 



Journ. Q. M. C, Series IT. — No. 65. 36 



