and an Optical Method of Calibration. 



427 



full intensity and to one half of this, to find any intermediate 

 values from the screw-readings, by simple proportionality, 

 assuming the screw itself to be accurate. Except in serious 

 disarrangement of the instrument this calibration can be relied 

 upon in all future measurements, even when new incandescent 

 lamps are used before the standardized slit of T'. 



Description of the Instrument. 



Fig. 5 shows the complete spectrophotometer as constructed 

 by Schmidt and Haensch, of Berlin, one-seventh actual size. 

 The two collimators, T and T', and the telescope, R, are 



Fiff. 5. 



mounted in one plane. The collimator T carries a unilateral 

 slit and is fixed rigidly on a radius of the ring Q. The 

 collimator T' carries a bilatei-al slit, which is calibrated and 

 used in making the settings, and is mounted on a radial arm 

 movable about the axis of the instrument and having a 

 micrometer screw W , and may be clamped to the ring Q. 

 Set screws may displace the screw W. The telescope K 

 is also mounted on a radial arm so that its axis passes 

 through the axis of the instrument. It can be clamped to 

 the ring Q, and carries a micrometer screw, N, and a vernier 

 for reading the divided arc, M, concentric with the axis of 



