o2 PROCEEDINGS OP THE AMERICAN ACADEMY 



the appearance of the mirror from the same amount of arsenic 

 varies, and the variation is more marked as the mirrors decrease in 

 size. Then, for greater convenience in interpolation, mirrors cor- 

 responding to the half-hundredths may be made, so that the set which 

 I use contains the following amounts. 0.005, 0.01, O.Ol.o, 0.02, 0.025, 

 0.03, 0.035, 0.01, 0.045, 0.05, and 0.06. Above 0.06 the difference 

 between the mirrors is so hard to estimate with accuracy, that it is of 

 no advantage to make any above 0.06. Yet in the set which I have 

 photographed, and which is shown in the plate, the following are added 

 for comparison : 0.07, 0.08, 0.09, 0.10. 



When the set is not in use, it should be kept in the dark, and at 

 no time be exposed to direct sunlight. Although the tubes be sealed, 

 yet there is sufficient air inside to cause a rapid oxidation in direct 

 sunlight, especially with the lower amounts, and a film of arsenious 

 oxide results. By observing this precaution, the set can be kept a 

 long time without alteration. 



The readings of the mirrors are far sharper by transmitted than 

 by retlected light, and comparison should always be made by the 

 former. For this purpose I use, at the suggestion of Professor Hill, 

 a small tin box painted black on the inside and outside, and similar 

 in shape and construction to the old-fjishioned stereoscopic camera. 

 The height of the box is 20 cm. ; width, 7.5 cm. ^ length at top, 

 10 cm. ; length at bottom, 15 cm. The bottom is open, while the top 

 is covered, and fitted with two eyeholes, 2 cm. in diameter and 4 cm. 

 apart. The set of standard mirrors, as shown in the plate, is 

 mounted, by means of sealing wax, in blackened wooden frames, 

 18 cm. long by about 7 cm. wide, and 4 to 5 mm. thick. The inner 

 dimensions of the frames are 12 cm. by 4.5 cm. The bottom of the 

 box carries a rabbet of tin, on which the frames may be slid, thus 

 bringing the mirrors under the eyehole.-^. A second rabbet above 

 the first allows the introduction of a frame carrying the mirror to 

 be compared, which may thus be brought between any two of the 

 standards and compared just as in nesslerization. The box is 

 mounted over a white paper or plate, in front of a good light. 

 The calculation of the amount of arsenic in the area 0^ paper taken 

 follows from the determination of the amount in the aliquot part of 

 the solution. From this can be found the number of milligrams 

 per square meter, which, when multiplied by the factor, 0.0128, gives 

 the number of grains per square yard. 



