204 Dr. H. R. Wright on the Photometry of the 



both from the slit and from the diffuse reflecting plate. All 

 rays parallel to the optical axis of the collimator unite in one 

 straight line, which coincides with the slit, whilst all other 

 rays fall either to the right or to the left of it on to the 

 carefully blackened metal parts of the photometer, and 

 cannot possibly reach the eye. The selection of the focal 

 distance and the size of the lens are very important for the 

 results. 



The Adjustment of the Apparatus. — The arc-lamp (compare 

 figure) rests on a stand, which can be raised and lowered by 

 means of a cog-wheel. It is also movable in two directions 

 in a horizontal plane. A Zeiss condensing-lens spherically 

 corrected procures the parallel beam of light. A circular 

 hole (//) is cut in the box carrying the condensing-lens so 

 that the centre of it lies in its optical axis. It is used as a 

 stop and also for centering the arc of the lamp. The closed 

 end of the cardboard tube (i) contains three apertures, which 

 are of such dimensions that only the plates were illuminated. 

 To make the border of the dark parts of the instrument more 

 exact still, the movable screen (k) v r as used. The diffuse 

 reflecting plate (a) was gently pressed by two brackets, 

 fixed with springs, against the brass piece (/), which could 

 be levelled by means of three adjusting-screws. It stood on 

 a "Meyerstein" spectrometer, from which the collimator 

 and telescope were removed. The graduated circle (c) with 

 two verniers is read by means of the two microscopes. The 

 photometer rests on a sliding board, which is carried by the 

 part (?i). A weight of 10 kg. balances the whole, which 

 turns round the same axis as the plate (a). The angles are 

 read by the large graduated circle (d) (diameter 51 cm.) and 

 the dials (r/ : ) and (d 2 ). The directions of the condensing- 

 lens-, cylinder-lens-, and collimator-axes must of course meet 

 in the axis of rotation, which must also pass through the centre 

 of the little plate («'), the mirror (/), and must lie in 

 the surface of the plate (a). A specially contrived body (r), 

 made of tin, prevents any extraneous light from coming into 

 the photometer, and carries the above mentioned cylinder- 

 lens (g) and plate (a'). The slit {{') sends light on to a 

 small mirror whose surface lies in the axis of rotation and which 

 is fixed on the brass piece (I). Thus one is enabled to 

 observe whether the angle between the incident ray and 

 the zero of the graduated circle is actually 90°. 



The Photometric Measurements. — Two different methods 

 are at our disposal : 



1st Method. — The length of the plate with regard to the 

 dimensions of the photometer is constant. Or : the projection 



