384 NOTES. 



more than gum copal dissolved in turpentine. It is open to 

 question, however, whether the good results that he obtained 

 were not, in the main, due rather to the extreme care taken in 

 the process than to the cement itself. A fine pair of prisms 

 representing Ahrens' best work, with an aperture of about 

 3 J inches, have fortunately been secured for the Science Museum^ 

 South Kensington, together w^ith a fine collection of projection 

 preparations for the prisms. Ahrens did a great deal of work 

 besides that already referred to. As time advanced, however, 

 he had it borne upon him that the demand for his work was- 

 decreasing. The various optical elements upon which he had 

 Bpecialised became ultimately in such great demand that the 

 introduction of machinery for their manufacture was called for. 

 Then Ahrens discovered, to his sorrow, that his erstwhile cus- 

 tomers preferred to go abroad for these optical appliances, 

 where they could obtain them cheaper and probably better made. 

 Personally Ahrens w^as a man of strong and independent 

 character — strictly honest in all his dealings, and, poor though 

 he was, autocratically selecting his own customers. His death 

 leaves a vacancy which it will be difficult to fill. 



Frederic J. Cheshire. 



