[ - 777 J 



LXXXV. An Interferometer for Testing Camera Lenses. 

 By F. Twymak, F.Inst. P. (Research Department, Adam 

 Rilger, Ltd.)*. 



[Plate XXIV.] 



INTERFEROMETERS for the testing and correction of 

 prisms, and of lenses (for axial pencils), have been 

 described by the author in the Philosophical Magazine f, and 

 in the patent specifications under which they are protected 

 in this and foreign countries. The subject is also dealt with, 

 and the principles of the camera-lens interferometer outlined, 

 in the 21st Traill Taylor Memorial Lecture J, where a number 

 of photographs of the interference effects shown by camera 

 lenses are given. 



The object of the present paper is to give a detailed de- 

 scription of a Camera Lens Interferometer recently made for 

 and despatched to an important optical firm by Adam 

 Hilger, Ltd. It is intended that it should form the standard 

 model of this instrument. 



A very brief description, therefore, of the author's earlier 

 interferometer for the testing and correctu n of prisms will 

 suffice for the purpose of this paper. 



The Prism Interferometer. 



This instrument in its simplest form resembles the w r ell- 

 known Michelson interferometer, the main essential optical 

 difference being that the two interfering beams of light are 

 brought to a focus at the eye of the observer. It is shown 

 in diagram (fig. 1) as arranged for the correction of a 60° 

 prism, such as is used for spectroscopy. 



The light used must consist of a limited number of very 

 homogeneous radiations. Such a light may be obtained from 

 a Cooper-Hewitt Mercury Vapour Lamp. The particular 

 radiation for which it is desired to make a test is isolated by 

 a suitable filter. 



The light from the source is reflected by the adjustable 

 mirror A through the condensing lens B, by means of which 

 it is condensed on the aperture of the diaphragm C. The 

 diverging beam of light is collimated by a lens D, and falls 

 as a parallel beam on a plane parallel plate K, the second 

 surface of which is silvered lightly so that a part of the light 

 is transmitted and part reflected. The major part should be 



* From an advanced proof of the Transactions of the Optical Society* 

 vol. xxii. No. 4, communicated bv the Hon. Secretary, 

 t Phil. Mag. xxxv. p. 49 (Jan'.' 1918). 

 % Printed in the Photographic Journ., Nov. 1918. 



Phil. Mag. S. 6. Vol. 42, No. 251. Nov. 1921. 3 F 



