14 STUDY OF ABSORPTION SPECTRA 



the calibrated drum-head one division towards the red would increase 



o 



the wave-length of light about 40 Angstrom units for the region which 

 we studied. Even this slight increase in wave-length would often 

 change the percentage transmission as much as 10 to 20 per cent. The 

 tabular data bring this out very clearly. It is therefore necessary to 

 exercise great care in setting the calibrated head which rotates the glass 

 prism. Table 2 contains data which show the effect of increasing 

 the slit-width, the changes that take place when the conditions for 

 Beer's law are fulfilled, and the accuracy which is possible in setting 

 the calibrated head of the spectroscope and in reading the deflections. 

 We could not read the deflections closer than 0.5 mm., and in the 

 visible red, with 0.2 mm. slit-width, the deflections were so small that 

 the error in reading may be as much as 2 per cent. For conditions 

 such as are indicated by this table, the minimum deflection obtained 

 for a slit-width of 0.2 mm. was 12 mm.; that for 0.4 mm. slit-width 

 was 63 mm.; the maximum deflection for 0.2 mm. slit-width was 

 69 mm. ; that for 0.4 mm. slit-width was 378 mm. 



We desired to use slit-widths as large as possible so as to increase 

 the accuracy of the readings ; this, however, could be done only when 

 the wider slit-widths had but little effect on the percentage transmission. 



It is desired to call attention to the duplicate results, which, in 

 the tables, are bracketed. For each duplicate measurement the head 

 of the spectrometer was reset and the deflection again read. The 

 second reading was made after the series of readings had been com- 

 pleted, and then, after completing the second series of readings a third 

 reading was taken for a few wave-lengths of light. The method of 

 procedure was as follows : It was desired to measure the absolute trans- 

 mission of 2 mm. of a 0.586 normal solution of neodymium chloride in 

 water. This was done by the differential method already discussed. 

 We place in cell 1, 3 mm. of the solution and note the deflection as given 

 by the radiomicrometer. Similarly, we place 1 mm. of the solution in 

 cell II and note the deflection when cell II occupies exactly the same 

 position formerly occupied by cell I. The deflection given by 3 mm. of 

 the solution, divided by the deflection given by 1 mm. of the solution, 

 gives the absolute percentage transmission of 2 mm. of the solution. 



The solution was diluted eight times, its concentration then being 

 0.073 normal. To satisfy the requirements of Beer's law we would 

 have to place in one cell 24 mm. of the more dilute solution and in the 

 other cell 8 mm. We would then have the same number of absorbers 

 in the path of the beam of light as we had in the case of the original, 

 more concentrated solution. In the case of the more dilute solution 

 we would obtain the absolute transmission for 16 mm. depth of layer. 

 This would be comparable with the transmission obtained for 2 mm. 

 of the more concentrated solution. Column 1 in table 2 gives the 

 actual head-readings. In columns 2 and 3 are given the percentage 



