for Testing 'Camera Lenses. 703 



point has been carefully attended to. It is desirable that the 

 ring system should be as large as convenient in order that the 

 amount of light admitted to the apparatus, and eventually to 

 the eve, may be as great, as possible, without making the 

 bands indistinct. The closer the convex mirror is to the 

 camera lens, the more nearly is this condition fulfilled. For 

 this reason a range of mirrors is provided which enables, for 

 anv lens, a mirror to be chosen which will give ample illumi- 

 nation for observation. 



Photography of the Appearances. 



The first difficulty in connexion with the photography of 

 the appearances is that with the changing temperature of air 

 and apparatus there is a slow movement of the interference 

 bands. Although this is in no way detrimental to ordinary 

 observation, since in spite of this movement the bands retain 

 their same general shape, yet, during the exposure necessary 

 for photography of the bands, their gradual movement may 

 produce a general fogging of the plate. For this reason it is 

 desirable when photographing the appearances to use a 

 silica tube mercury-vapour lamp. With such a lamp photo- 

 graphs can readily be obtained, either for green or violet 

 radiation, with a 2-minute exposure. With a suitable camera, 

 therefore, in which five exposures can be given successively 

 on one long plate, a complete photographic record can be 

 obtained of the performance of any camera lens in a very 

 short time. For example, if the observer be given for test a 

 photographic lens he can within one hour hand the lens back 

 with a record and photographic plate affording the following- 

 information. 



(a) Focal lengths of the lens correct to 0*001 in. for the 

 green and violet rays. 



(b) Complete aberrations (including distortion, and the 

 chromatic variations of aberration) for axial pencils and for 

 five obliquities. 



Fig. 16 (PI. XXIY.) is a photographic reproduction of the 

 inter ferograms of a well-known lens of high repute for the 

 green mercury radiation (54G fifi) for the axial beam and for 

 obliquities of 5, 10, and 15 degrees. It will be seen that 

 even the best photographic lenses— of which this is a fair 

 example — are very far indeed from perfection. 



I must not fail to record my thanks to Mr. Dowell and 

 Mr. Perry, members of the technical staff of Adam Hilger, Ltd, 

 To the former is due the detailed design of this Interferometer, 

 while the latter has carried out all the adjustments, and taken 

 the photographs reproduced in PI. XXIV. fig, 16. 



Phil. Mag. S. 6. Vol. 42. No. 251. Joe 1921, 3 G 



