Examination of Photographic Lenses at Kew. 



405 



decided on the size of the plate he intends to use, and that what he 

 wants to know is whether it is suitable for that plate or not. When 

 further information is desired, the lens may be entered for examina- 

 tion for two or more sizes of plates. 



The following is an example of the Certificate of Examination, the 

 part in italics representing the result of the testing of the lens. 

 Reference is made to the pages where a detailed discussion on each 

 test can be found : 



KEW OBSEEVATOBT, RICHMOND, STJKBEY. 

 Certificate of Examination of a Photographic Lens. 



1. Number on lens, 3876. Registered number, 95. 



2. Description, landscape lens. Diameter, I'd inches. 



3. Maker's name, A. JB. 



4. Size of plate for which the lens is to be examined, 6'5 inches by 8'5 inches. 



5. Number of reflecting surfaces, 4 (see ? 409) . 



6. Centering in mount, good (see p. 409). 



7. Visible defects such as striae, veins, feathers, &c., nil (see p. 409). 



8. Flare spot, nil (see p. 409). 



9. Effective aperture of stops (see p. 412) 



10. Angle of cone of illumination with largest stop = 68, giving a circular image 



on the plate off 13' 2 inches diameter. 



Angle of cone outside which the aperture begins to be eclipsed, with stop C.I. 

 No. ljl'38, = 2 0, giving a circular image on the plate of IfO inches dia- 

 meter. 



Diagonal of the plate = 10'7 inches, requiring a field of 51. 



Stop C.I. No. 5'3 is the largest stop of which the whole opening can be seen 

 from the whole of the plate (see p. 414). 



11. Principal focal length,f = 11'28 inches. Back focus, or length from the prin- 



cipal focus to the nearest point on the surface of the lenses, = 10'4 inches 

 (see p. 418). 



12. Curvature of the field, or of the principal focal surface. After focussing f the 



plate at its centre, movement necessary to bring it into focus for an image 1'5 

 inches from its centre = 0'02 inch. 



* C.I. International Congress System. 

 accompany certificate.] 



[See p. 413; an explanatory note will 



f The lens is focussed on a very distant object. 

 VOL. LII. 



