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LXXIX. A Modified Form of Double Slit Spectrophotometer. 

 By A. L. Naeayan, AJ.A* 



[Plate XIV.] 



IN different kinds of spectrographs work which have been 

 carried on in the Physical Laboratory of the College, 

 the need for a good spectrophotometer in the visible and 

 ultra-violet regions is keenly felt for the exact location of 

 the absorption centres and for the measurement of their 

 intensity. One of the spectrophotometers adapted to the 

 purpose is the one designed by Vierordt. In this,, the colli- 

 mator is furnished with a double slit in front of which the 

 absorption cell is placed in such a position that the dividing 

 line between the two layers of different thicknesses or the 

 dividing line between the liquid and air coincides with the 

 junction of the two slits. And the slit widths are adjusted 

 until equal illumination is obtained in both the spectra. But 

 the difficulty in the way of regulating the brightness of the 

 spectrum by altering the width of these slits is that it 

 seriously interferes with the purity of the spectrum. 



A modified form of double slit spectrophotometer has been 

 devised, which is free from the defects of Vierordt'' s type. 

 At the same time, it possesses many of the advantages of the 

 sector photometer. 



It consists essentially of an electromagnetically maintained 

 pendulum the period of vibration of which is '5 second 

 nearly, so that it executes 120 oscillations per minute. .The 

 bob of the pendulum is a heavy frame of brass containing 

 two rectangular apertures one above the other, there being a 

 Avell-defined line of demarcation between the two. 



Two photographs are given, of which No. 1 shows the 

 photometer and its parts, and No. 2 the photometer in position 

 before the spectrograph. 



The widths of both the slits can be adjusted by independent 

 micrometer screws. The whole pendulum is mounted in 

 front of the collimator slit of the spectrograph such that the 

 line of separation between the two slits almost bisects the 

 collimator slit. And the bob of the pendulum can be raised 

 or lowered by a small amount by working a nut by means of 

 which the whole frame constituting the bob is fixed to one 

 end of the pendulum rod. 



Since the period of the pendulum is *5 second, the whole 



* Communicated by the Author 



