NUTTING AND JONES: PHOTOMETER FOR SMALL AREAS 315 



latter pieces are mounted on an arm carried by the instrument 

 itself, the lens Lx being adjustable in position. 



In use, the sector S is first set to read 100 and the source E 

 moved toward or away from C until a balance is secured with- 

 out the plate to be measured. The instrument will then read 

 percentage transmission from nothing to 100 per cent directly, 

 with but a very slight zero correction. 



For determining reflecting powers, Ri and D are not used and 

 Ro is rotated to face downward (instead of horizontally, as shown) . 

 The surface upon which observations are to be taken s placed 

 directly under R^ on the table and the lens Li focussed upon the 

 surface which is illuminated directly by the source E at an angle 

 of about 45? Ordinarily, it is diffuse reflecting power that is 

 desired, and the surface is placed horizontal — -specularly reflected 

 light not entering the instrument. 



The zero adjustments for measuring reflecting powers are sim- 

 ilar to those made in measuring transmissions. If only relative 

 values are desired, as in measuring up a photographic print, 

 clear paper is placed in the field, the sector set to read 100 per 

 cent and the som'ce E adjusted to give a match at the cube. 

 If actual reflecting powers are desired, a mat surface, say a 

 block of magnesium carbonate, whose reflecting power has been 

 determined on an absolute reflectometer,^ is placed in position, 

 the sector S to read its absolute reflecting power, and the source 

 E adjusted as before. The instrument will then give directly 

 the diffuse reflecting power of any other surface placed in the 

 field. If specular reflecting power is to be determined, total 

 reflecting power is measured on the absolute instrument and 

 diffuse reflecting power measured as just described. 



The instrument, as a whole, is easily, quickly, and perma- 

 nently adjustable; readings may be taken with it as rapidly and 

 accurately as with any kind of visual photometer, and we have 

 not yet detected any systematic errors in its readings. For 

 months it has been in constant use by a mmaber of observers in 

 the sensitometry of photographic plates and papers. With a 

 magnifying ocular it has even been used as a microphotometer 

 of low power. 



1 P. G. Nutting, Journ. Wash. Acad. Sci. 2: 505. December 19, 1912. 



