Examination of Photographic Lenses at Kew. 



451 



with one only covering 40, in order to get the same mean exposure 

 over the whole plate. 



In connexion with this test it may be mentioned that the most 

 serious omission in the Kew examination is, that there is nothing to 

 show the actinic transparency of the glass. A slight yellow tinge in 

 the lenses, which would not be noticed by the eye, might yet be 

 sufficient to seriously affect the rapidity of the objective. But no 

 test could be devised to investigate this point which did not intro- 

 duce photographic methods, and, as already stated, the consideration 

 of expense put such operations out of consideration for the present. 

 I should like, if possible, to have introduced some test which would 

 have at the same time indicated the actual rapidity of the lens, and 

 also the actual falling off of density towards the margin of the photo- 

 graph ; with the aid of photography this would not have been 

 difficult, and a plan of this kind would have been adopted, but for 

 the cost. This subject is, however, still under consideration by 

 Captain Abney. 



