NOTES. 383 



struction of a prism binocular field glass was at that time little 

 understood, and has not been as yet pointed out in any treatise 

 on optics. The curves of the objectives must be altered so as to 

 balance the undercorrection caused by the prism. Prof. Abbe saw 

 that this undercorrection was fully compensated for, and there- 

 fore the^e glasses were from the first a great success. Now with 

 regard to the Greenough binocular. The twin microscope bino- 

 cular was invented by Pere Cherubin d'Orleans in 1677, but as 

 the microscopes were of the ordinary inverting form the image was 

 pseudoscopic. To remedy this, erecting eye-pieces of Eheita's 

 pattern were added; these certainly changed the pseudoscopic 

 vision to orthoscopic, but owing to the great increase in the length 

 of the body the inclination of the tubes to one another was so 

 much reduced that very little stereoscopism was left in the image. 

 A twin-microscope binocular must have short bodies, and it was 

 this microscope of Ahrens which made the twin-microscope 

 binocular a practical instrument. 



Edward M. Nelson. 



Another link with the past has just been broken by the death 

 of Carl Dietrich Ahrens at the ripe age of eighty-one. A skilled 

 handicraftsman with a distrust of all machinery for the doing of 

 his beloved work, he was a typical representative of the pioneer 

 artists who so often labour so diligently to lay the foundations 

 of a new industry, only to find that the more successful their 

 work, the more unnecessary their own activities ultimately 

 become, as labour-saving machinery is introduced to meet the 

 demands their own work has created. 



Ahrens' best work was undoubtedly done in the making of 

 the large Nicol prisms which have been so closely identified 

 with his name for the last half-century. At such work he was 

 an artist. Possessing but a very limited amount of theoretical 

 knowledge, his instinctive knowlege of the optical ejects of 

 Iceland spar, quartz and other double-refracting media, when 

 €ut and combined in various ways, was wonderful; and no one 

 had quite the success in the important operation of cementing 

 such prisms that he had. For many years, in fact until just 

 before his death, the cement employed by him was kept as a 

 great secret. It is now known, however, that he used nothing 



