for Testing 'Camera Lenses. 783 



back through the lens is adjustably mounted on a carriage 

 and the carriage is automatically moved towards or away 

 from the lens, so that the centre of curvature of the mirror 

 is always in the focal plane in which the lens is supposed to 

 form its image. This is accomplished by a flexible connexion 

 being led from the carriage to a weight, and by there being 

 upon the carriage a roller which is retained in contact with 

 the crossbar above mentioned by the action of the weight. 



Light from a suitable source is reflected by a mirror, 10 

 (fig. 6), and condensed by a lens 11 on to the aperture of a 

 diaphragm 12 through which it passes. The diaphragm is 

 so mounted that it can be readily interchanged with another 

 of different aperture if desired. After passing through the 

 aperture the light is rendered parallel by a lens, 010, and 

 passes to a plane glass plate, 008. The surface of the glass 

 plate is partially silvered so that part of the light is reflected, 

 and part transmitted by it. The transmitted light passes on 

 through a pair of deflectors, 011, by means of which the 

 light can be deflected in a horizontal plane, the deflectors 

 being rotated in opposite directions by the milled head, 212. 

 The deflexion is measured by a scale on the drum 204 and 

 index 206. The light passes onwards to the lens 14 which is 

 under test and to a spherical mirror, 001, by which it is 

 reflected back through the lens under test and the deflectors 

 011 to the plate 008 and is partly reflected to the lens 009, 

 by which an image of the aperture in diaphragm 12 is formed. 

 The part of the beam which is reflected by the plate 008 

 proceeds to the mirror 007, by which it is reflected back 

 again. A part is transmitted by the plate 008, and combines 

 with the beam returning through the lens 114. 



The lens under test is mounted on a cross slide whereby it 

 can be adjusted either along or at right angles to its optical 

 axis, and in this way its second principal point can be brought 

 on to the axis about which it is rotated by the bar 105 (figs. 6 

 and 10). The spherical back reflecting mirror 001 is mounted 

 in a cell, 145 (fig. 8), which fits the adapter 142 and is 

 clamped by the turn-buttons 152. Cells, each containing a 

 mirror of different radius, are thus readily interchangeable. 



The adapter 142 is pivoted on a ball resting in a coned 

 recess, and is kept in contact with it by the spring 156 and 

 three rods, of which two 146 and 154 are shown. The spring- 

 box, 143 (figs. 8 and 9), to which one end of the rod 146 is 

 attached, also forms the end of a lever against which the 

 horizontal and vertical slow-motion screws, 159, and spring 

 plungers, 144, act. The centre of curvature of the mirror 

 can thus be adjusted in a horizontal and a vertical plane. 



The mirror pivot-bearing, 157, and slow-motion are mounted 



