1 6 MEASUREMENT OF ANGLES. 



The use of a lens is especially advantageous in apparatus for 

 variation when we desire to keep as a mark the image furnished 

 by a fixed mirror : in this case it is in fact more convenient to 

 take a plane mirror. 



674. PROJECTION OF IMAGES. For routine experiments the 

 method of reflection is greatly simplified by dispensing with the 

 telescope, and projecting directly upon the scale the reflected 

 image of a luminous object. 



In the ordinary arrangement, the plane of the scale and that 

 of the cross wire of the telescope are conjugate in reference to the 

 system formed by the object-glass and the mirror. If then, the 

 eye-piece being removed, the cross wire is illuminated, its image 

 will be projected on the scale at the very division which would 

 have been read on the telescope. The cross wire may then be 

 replaced by a vertical slit illuminated from behind. 



The use of a concave mirror gives a still more simple arrange- 

 ment. The scale and the illuminated slit are placed in the same 

 plane, which passes through the centre of curvature of the mirror 

 in other words, at twice the focal distance, and at the same distance 

 on either side of this centre in a vertical direction. The image 

 of the slit is then thrown on the scale in its proper size. 



A plane mirror and lens, placed at a small distance, may be 

 substituted for the concave mirror. The slit must then be placed so 

 that its image is sharply formed on the scale. 



In this method of reading by projection the external light must 

 be shaded in order to see the image of the slit on the scale ; the 

 reading then becomes difficult. This inconvenience is obviated by 

 taking a very large slit or circular aperture, across which wire is 

 stretched vertically. The image of the aperture is then produced on 

 the scale, and illuminates the space in which is the image of the wire. 



Very good results are obtained with a scale on ground glass, 

 which is observed from behind. 



The method by projection, which allows a photographic impres- 

 sion of the image to be taken, is that used in registering apparatus 

 for variations of terrestrial magnetism. By using two mirrors, one 

 fixed and the other movable (670), we get on the sensitive surface 

 two images of the same illuminated slit one fixed, which determines 

 the zero, and the other changing its position, which measures the 

 variation. If the sensitive layer moves in a direction perpendicular 

 to the displacement of the image, we obtain in the proof a right line 

 corresponding to the fixed image, and a curve which represents the 

 movable image. 



