82 ROBERT M. YERKES AND JOHN B. WATSON 



purchased through the Johns Hopkins University. It will be 

 sent out after being standardized, with a chart and full directions 

 for installation. 



Spectrophotometers. — While the measure of energy in the 

 bands of monochromatic light seems to be the most desirable 

 form of control, since it is absolutely independent of the sen- 

 sitivity of the human eye, the apparatus for obtaining it. as 

 we have seen above, is expensive, and the technique is some- 

 what difficult. It seems worth while, accordingly, to mention 

 at least another standard method of control — that of spectro- 

 photometry. This method, as is well known, measures the 

 intensity of a variable monochromatic beam in terms of the 

 intensity of a monochromatic beam of the same wave-length 

 coming from a standard source. There are many complex 

 forms of spectrophotometers on the market, such as the Lummer- 

 Brodhun, costing about $500, the Brace, costing $375, Glan's, 

 costing $125, but all these are more complicated than is necessary 

 for our w^ork. 



After some experimentation we suggest the use of Nutting's 

 pocket spectrophotometer. This is a compact little instrument, 

 which gives an accuracy within 2-3 per cent, of the larger in- 

 struments. It is inexpensive ($31) and easy to use. It is made 

 by R. Fuess, and may be purchased through the Arthur H. 

 Thomas Company, Philadelphia. 



The instrument needs a small tungsten lamp of known candle 

 power attached to it at a fixed distance from the slit, e. g., 10 

 cm., for the purpose of giving a reference or standard spectrum. 

 The instrument should be mounted in a metal arm running at 

 right angles to it for convenience in clamping it to an upright. 

 This arm is graduated in mm. and carries the standard lamp. 

 The white light from this lamp enters the instrument through 

 a window in the side, meets there a small total reflection prism, 

 and is reflected through the sHt parallel to the axis of the in- 

 strument; passing through the usual lenses, direct vision prism 

 and Nichol prism (the polarizer), it is refracted, polarized and 

 finally focussed on the vertical slit in front of the eyepiece. The 

 width of this slit is adjustable. The variable monochromatic 

 beam enters at the end of the instrument, passes through the 

 first Nichol prism (the "analyzer"), thence through the lenses, 

 the second Nichol, etc., as above. In the eyepiece one spec- 



