288 Transactions of the Society. 



micrometer* into the linear distance between the lines which give, 

 as viewed by the unaided eye, a retinal image of similar dimensions. 

 In conclusion, I desire to express my thanks to Messrs. 

 R. and J. Beck, to Messrs. Swift and Sons, and to Messrs. Sanger 

 Shepherd, for the eikonometers and the diffraction gratings (ruled 

 400 lines to an inch) which have served for the illustration of the 

 methods I have brought before you. 



* The more widely interspaced lines of the central ruling of an Abhe diffraction 

 grating furnish a very convenient stage-micrometer for the procedure here described. 



