LIGHT MEASUREMENTS 843 



even physicists like Vierordt (1871) and Lazarev (1924, 1927)— thought 

 that they could neglect it. In most measurements, however, an attempt 

 was made to include at least the diffusely transmitted light, Ta, by the 

 simple device of placing a large collecting surface immediately behind 

 the absorbing system. Seybold (1932) pointed out that this procedure 

 brings the risk of measuring the thermal radiation of the tissue together 

 with the transmitted flux. (A similar error could be caused by fluorescence, 

 but the latter usually can be neglected.) To avoid errors, one may inter- 

 pose an infrared-absorbing filter between the leaf and the collecting ther- 

 mopile. 



The measurement of the diffusely reflected flux Ed requires more elabo- 

 rate devices and has often been omitted. The resulting error in the deter- 

 mination of the absorbed intensity can be considerable, since leaves of 

 land plants reflect about as much, or more, light as they transmit — namely, 

 from 10 to 15,% of (infrared-free) white light (c/. page 683). Submerged 

 algae or water-filled leaves have a lower reflectance — they transmit about 

 20% and reflect from 5 to 10% of white light. However, diffuse reflection 

 cannot be entirely neglected even when working with algal suspensions, as 

 shown by the results of Noddack and Eichhoff (1939) in figiu-e 22.2. The 

 sharp reflection peak at 180° is due to the walls of the vessel; but, in addi- 

 tion to this specular reflection, the figure shows a small, but not negligible, 

 diffuse reflection ; integrated over all angles, it adds 3 or 5% to the trans- 

 mitted flux and reduces correspondingly the absorbed energy, A. 



In work with cell suspensions in spherical or cylindrical vessels, the dis- 

 tinction between reflected and transmitted light becomes irrelevant, and an 

 integral measurement of light scattered in all directions, S, can be sub- 

 stituted for the separate measurements of R and T. A small vessel con- 

 taining the suspension can be placed inside an "integrating" box or cell, 

 or in the focal point of a mirror, illuminated by a narrow beam of light 

 entering through a hole in the mirror, and the light scattered in all direc- 

 tions can be collected and measured. For the determination of /, a "white" 

 scatterer can be substituted for the suspension cell. A device of this type 

 was Noddack and Eichhoff's (1939) "ellipsoid photometer," in which the 

 light scattered by a small cell was collected on a thermopile sensitive to 

 light falling from all directions. The scatterer was placed at one focus of 

 an ellipsoidal mirror, and the collector at the other. 



In speaking of the methods of determining the hght energy absorbed by cell suspen- 

 sions, we must also mention Warburg and Negelein's method of total absorption (1922, 

 1923). These authors used a very concentrated Chlorella suspension; the vessel had a 

 silvered back wall so that no light was transmitted; and the absence of diffusely re- 

 flected hght was ascertained by experiment. (The correctness of this last assertion was 

 questioned by Mestre 1935, and this criticism is supported by the above-mentioned re- 

 sults of Noddack and Eichhoff.) Thus, \\'arl)urg and Xogelein assumed, simply, A = / 



