44 



THE AMERICAN MONTHLY 



[March, 



to either take in a larger bundle of 

 rays from the object, which would in- 

 crease the relative brightness of the 

 latter, or a larger bundle from the 

 paper, when the relative brightness 

 of the object would be reduced. In 

 this way it is possible to follow details 

 of an object very accurately with the 

 pencil. Consequently, it is not nec- 

 essary to adjust the light so perfectly 

 when using the Zeiss instrument, and 

 if the illumination of the object be 

 quite strong, the eye can still follow 

 the pencil-point. Indeed, it is some- 

 times desirable to make the ligrht on 



pie reflecting disk over the ocular 

 which would throw an image of pen- 

 cil and paper into the eye, the object 

 being seen over the edge of the disk, 

 would be a type of the best possible 

 form, 



A few words now concerning the 

 distortion produced by the two forms 

 mentioned. A stage-micrometer ruled 

 in hundredths of an inch, and a §-inch 

 objective, were used. 



The Grunow instrument was first 

 used, the inclination of the stand 

 being, in two different experiments, 

 30° and 40° from the vertical. 



Fig. II. — Zeiss Camera Lucida. 



Fig. 12. — Grunow Camera Lucida. 



the object very strong, for then many 

 details can be drawn which are not 

 visible when the pencil is also seen, 

 but by properly placing and moving 

 the eye, the pencil can be seen just 

 behind the sharply defined border of 

 the bright image, and made to follow 

 after it, in a manner not easy to de- 

 scribe. 



Owing to the readiness with which 

 the eye equalizes the illumination of 

 paper and object in camera lucidas in 

 which the light is divided in this way, 

 I am inclined to regard them as theo- 

 retically the most perfect form, and, 

 speaking from theory and not from 

 from practice, I should think a sim- 



1. Inclination 30°, lines of the mi- 

 crometer running vertically across the 

 field. 



The diameter of the field projected 

 upon the paper was, approximately, 

 seven inches. The distance between 

 the lines, near the margin of the field 

 nearest the microscope on the paper, 

 was 1.03 inches; at the margin furth- 

 est from the stand it was 1.15 inches. 

 Hence the difference in magnification 

 at the two extremes of the field was 

 12 diameters. 



2. Inclination 40°. Repeating the 

 same experiment under this inclina- 

 tion, the results were respectively 95 

 and 100 diameters; difference, 5. 



