SELIG HECHT 5 



3. In order to follow quantitatively the progress of the bleaching 

 reaction, it is necessary to have a series of colorimetric standards rep- 

 resenting different concentrations of visual purple and its decomposi- 

 tion products. Eleven such concentrations are made up, representing 

 on the one extreme 100 per cent of the unbleached substance plus 

 per cent of the bleached; and on the other extreme, per cent of the 

 unbleached plus 100 per cent of the bleached, the concentrations 

 changing by 10 per cent steps. Actually these are made up in dry 

 test-tubes by taking a total of 20 drops of solution: the first tube 

 having 20 of the unbleached; the next tube 18 of the unbleached plus 

 2 of the bleached; the next 16 of the unbleached plus 4 of the bleached, 

 etc. The same dropping pipette is used each time to insure uniformity. 



The experimental unbleached solution is pipetted into the small 

 exposure tubes. Into similar tubes and under the same conditions 

 are pipetted the standard concentrations just described. The stand- 

 ards are jacketed with thick, black rubber tubing, except for a few 

 millimeters at each end. In this way a much clearer end-on view is 

 obtained. The exposure tube containing the experimental solution 

 of visual purple is similarly jacketed when colorimetric comparisons 

 are to be made. The matching is done against an artificial daylight 

 lamp of 0.04 candle power, having a uniform circular surface 30 mm. 

 in diameter. Such a light is sufficiently bright for work in the dark, 

 especially after one has become dark-adapted. The matching light 

 is turned on and off by a spring contact controlled by the observer's 

 foot, so that only a momentary exposure is made. 



When not in actual use the standards are kept under cover even 

 in the dark room. As a rule, fresh standards are made daily. If they 

 have been used but little, they may be kept on ice until the next day, 

 provided they are placed in a moist atmosphere so as to prevent evap- 

 oration. They suffer comparatively little deterioration from their 

 use, mainly because the exposures to the daylight lamp are so brief. 

 After continuous use for several days, they are definitely oft"; hence 

 the practice of making fresh standards daily. 



4. The experimental tube is exposed to light of known intensity. 

 The apparatus used for this purpose is shown full size in Fig. 1 . The 

 glass rod to which the exposure tube is attached is rotated by hand 

 at a rate depending on the intensity of the light. For the experi- 



