74 ROBERT M. YERKES AND JOHN B. WATSON 



air-space between. Four such lenses have already been ground 

 to our order. They are very satisfactory. 



After this lens has been installed it should be carefully pro- 

 tected from a possible discharge of particles from the arc by a 

 metal cap. The danger from the arc comes when it is first 

 started and when the carbons have " frozen " and suddenly 

 burn apart. After the arc has begun to burn steadily the cover 

 can be removed. There is little or no danger of breakage from 

 heat. 



A compound portrait lens is best for the collimator. The 

 Voitlander, if it can be obtained with short focus, is very satis- 

 factory. There are several of these on the " second-hand " 

 market. They should not be over 12" in focus nor under 3" 

 clear aperture. 



The objective should be of the compound type if possible. 

 A large compound lens from a camera or stereopticon can be 

 used. The clear aperture should be at least 3^' in diameter 

 and the focal distance not over 50 cm. Several Steinheils may 

 be obtained which are accurate enough for all the work recom- 

 mended in this report. They may be had of Robey-French 

 and Company, Boston, or from Chas. G. Willoughby, 814 Broad- 

 way, New York. 



It is well to test the lens before purchasing, by mounting it 

 as a camera and allowing the image of a series of long, accu- 

 rately ruled cross lines to fall on a ground glass plate. These 

 lines should all appear in the image equally intense and undis- 

 torted. Many of the compound portrait lenses on the market 

 give images which are not sharply defined. This " softness " 

 in definition is an advantage in photography but it is a serious 

 defect in an objective for optical work. 



Several small achromatic projection lenses are needed — three 

 behind the double slit, one for the box containing the Nernst 

 filament (see p. 50) and one as a condenser w^hen the Nernst 

 is used in place of the arc. For these purposes we have used 

 small field or opera glasses. Opera glasses costing $3 or $4 a 

 pair will afford suitable lenses. They should be about 3^" to 

 4" in focal length and should have an aperture of about i". 

 They are to be entirely removed from their mountings and 

 remounted as shown in fig. 9. The Wollensak Optical Com- 

 pany, Rochester, N. Y., will grind small single achromats to 



