Major L. Darwin. On the Method of [June 2, 



I. "On the Method of Examination of Photographic Objectives 

 at the Kew Observatory." By Major L. DARWIN. Com- 

 municated by Captain ABNEY, R.E., F.R.S. Received 



April 13, 1892. 



(Abstract.) 



The paper describes the method of examination of photographic 

 objectives which has been adopted at the Kew Observatory, chiefly 

 on the recommendation of the author. In selecting and devising the 

 different tests, Major Darwin acted in co-operation with Mr. Whipple, 

 the Superintendent of the Observatory, and was aided by consulta- 

 tions with Captain Abney. 



The object of the examination is to enable any one, on the pay- 

 ment of a small fee, to obtain an authoritative statement or certificate 

 as to the quality of an objective for ordinary purposes ; the amount 

 of time that can be devoted to each experiment is therefore strictly 

 limited. 



An example is first given of a "Certificate of Examination" snch 

 as would be obtained from Kew, and then the different tests are dis- 

 cussed in detail. Many of them are done by well-known methods, 

 which need not here be mentioned. For the greater part of the ex- 

 amination, an apparatus called the " testing camera " is employed, and 

 this is fully described in the paper. The following are the different 

 items in the Certificate of Examination, including the various tests to 

 which the objective is subjected, or the subjects about which in- 

 formation is given : 



(1.) to (4.) Under these headings are given a description of the 

 objective, the date of receipt, the size of the plate for which the 

 objective is to be examined, &c., &c. None of this information forms 

 part of the result of the testing. 



(5.) Number of Reflecting Surfaces of the Lenses. 



(6.) Centering in Mount. 



(7.) Visible Defects, such as Veins, Feathers. Sec. 



(8.) Flare Spot. 



(9.) Effective Aperture of Stops, which is given for each one sup- 

 plied with the objective. In recording the results, it is proposed that 

 the system of numbering recommended by the International Photo- 

 graphic Congress of Paris of 1889 should be adopted. 



(10.) Angle of Cone of Illumination, ^c. Under this heading 

 several useful items of information are given, such as the extreme 

 angle of the field which is illuminated by the objective without 

 eference to definition; the angle of field which is required to cover 

 the plate of the size for which the objective is being examined ; and 

 the largest stop of which the whole opening can be seen from the 







