406 



Major L. Darwin. On the Method of 



Ditto for an object 3 inches from its centre = P04 inch. 



f =0-15 " (see p. 425). 



13 Definition at the' centre with the largest stop, excellent. O.I. stop No. ^ 



TTJ2 definition over the whole of a 6'5 inch by *B inch plate (see p. 429) . 



14 Distortion. Deflection or sag in the image of a straight line which if there 



were no distortion, would run from corner to corner along the longest side of 

 a 6-6 inch by *5 inch plate = + Ml inch* (see p. 436) ,..,...', 



15 Achromatism. After focussingf in the centre of the field m white light, the 



movement necessary to bring the plate into focus m blue light (dominant 

 wave-length, 4420), = +<W inch.* Ditto in red light (dominant wave- 

 length, 6250) = -0-01 inchf (see p. 440). 



16 Astigmatism.! Approximate diameter of disc of diffusion! m the image of a 



point, with C.I. stop No. at inches from the centre of the plate 



= 0* inch (see p. 443). 



17. Illumination of the field. The figures indicate the relative intensity at different 

 parts of the plate.f 



With C.I. stop No. 111-38. 



Atthecentre 100 : Ditto 



At 3 inches from the centre 67 : Ditto 



At 5'35 - 28 : Ditto 



With stop No. 5'3. 



100 



66 



General Remarks. An excellent medium angle rapid objective, practically free from 

 distortion. 



Date of issue 



W. HUGO, Observer. 



G. M. WHIPPLE, Superintendent. 



* The sag or sagitta here given is considered positive if the curve is convex 

 towards the centre of the plate. 



f The lens is focussed on a very distant object. 



J Positive if movement towards the lens, negative if away from it. 



The lens is supposed to be perfect in other respects. 



Note. The following is the scale of terms used : excellent, good, fair, indifferent, 

 bad. 



In considering and in recording the results of examinations, it 

 has been found convenient to give more exact meanings to certain 

 expressions than have as yet been assigned to them. The following 

 definitions have therefore been adopted at Kew : 



A narrow angle lens means one covering effectively not more than 35. 



A medium angle lens means one covering between 35 and 55. 



A wide angle lens means one covering between 55 and 75. 



An extra wide angle lens means one covering more than 75. 



With regard to the wording of the " General Remarks " in the 

 certificate, it should be remembered that the lens is judged entirely 

 with reference to a plate of named size ; the lens is therefore classed 

 as above by the angle of field which is given as the last item but one 

 in test No. 10. If the same lens is examined for plates of different 

 sizes, the certificate would be worded differently in each case. 



