566 SUMMARY OF CURRENT RESEARCHES RELATING TO 



axis makes witli the vertical, for this angle is no other than that 

 which the apex of the triangle formed before it was reversed. 



The first of these methods is very easy to execute, but it requires 

 much time and patience on account of the numerous trials which it 

 necessitates. The second is very easy also, and much quicker, since it 

 gives at once the result sought. I therefore prefer it, but in order to 

 be quite sure that no error has been committed, it may be checked by 

 the first. 



When the plane of the drawing is very exactly perpendicular to 

 the optic axis, all the distortions will disappear. Equidistant lines 

 will remain so, squares will remain squares — in a word, the drawings 

 will be faithful copies of the objects. This correction is very striking 

 when we employ the drawing-board above described. If the board is 

 horizontal, all the distortions are produced; when it is suitably 

 inclined, they immediately disappear. 



We may therefore, without changing in any respect the camerse 

 lucidse of Milne-Edwards and Nachet, obtain with these instruments 

 drawings as exact as possible. 



Are these facts known? I believe so, and in a previous article* I 

 have been content to allude to them ; but having seen that they were 

 not noticed in our classical authors, I have thought it well to direct 

 the attention of microscopists to them." 



Mr. E. Hitchcock, in a discussion on the relative merits of the 

 Zeiss (double prism) and Grunow camerfe lucidfe, adds some 

 remarks t concerning the distortion produced by the two forms. A 

 stage-micrometer ruled in hundredths of an inch, and a 2-3rds in. 

 objective were used. 



The Grunow instrument was first used, the inclination of the 

 stand being, in two different experiments, 30° and iO° from the 

 vertical. 



1. Inclination 30°, lines of the micrometer running vertically 

 across the field. 



The diameter of the field projected upon the paper was, approxi- 

 mately, 7 in. The distance between the lines, near the margin 

 ' of the field nearest the Microscope on the paper was 1 * 03 in. ; at the 

 margin furthest from the stand it was 1*15 in. Hence the difference 

 in magnification at the two extremes of the field was 12 diameters. 



2. Inclination 40°. Repeating the same experiment under this 

 inclination, the results were respectively 95 and 100 diameters ; 

 difference, 5. 



Zeiss' camera lucida, inclination about 13°. As only half the 

 field was projected on the paper, the lines had to be extended across 

 to 7 in. to make the results comparable with those of the Grunow. 



3. Lines vertical. Magnification at the extremes of a field of 

 7 in. diameter, respectively 104 and 113; difference, 9. This is 

 greater than with the Grunow, but the actual distortion of vertical 

 lines produced by this camera is only 4 • 5 at the greatest. 



4. Lines horizontal. Owing to the small field visible, only two 



* Infra, p. 567. 



t Amcr. Mou. Micr. Jouni., iv. (1883) pp. 43-5 (2 figs.). 



