1883.] 



MICROSCOPICAL JOURNAL. 



45 



Zeiss camera lucida, inclination 

 about 13°. As onl)' half the lielcl was 

 projected on the paper, the lines had 

 to be extended across to seven inches 

 to make the results comparable with 

 those of the Grunow. 



3. Lines v'ertical. Magnification at 

 the extremes of a field of seven inches 

 diameter, respectively 104 and 113; 

 difference, 9. This is greater than 

 with the Grunow, but it will be shown 

 that 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 contiguous 

 spaces were measured, showing an in- 

 rease from the centre outward of 3 

 diameters. 



The above figures should not be 

 considered as absolutely accurate. 

 They were not made with a view to 

 publication, and are only approxi- 

 mately correct — sufificently so to illus- 

 trate the subject. 



The Grunow instrument shows the 

 entire field of view on the paper. 

 Measuring from the middle of the sur- 

 face of the prism to the margins of the 

 field on the paper, we find the angle of 

 view to be 16.5°, the margin nearest 

 the microscope being only 3° from the 

 vertical, the center being 17^ from that 

 margin, and 14° from the opposite 

 side. Hence, the distortion produced 

 diminishes slightly toward the center 

 from either side; but the real differ- 

 ence from side to side, is shown by 

 our results. 



The Zeiss instrument only gives 

 half the field ; but the centre is almost 

 directly beneath the centre of the face 

 of the prism, 3.5° from the vertical, 

 hence the distortion is about equal on 

 either side of the centre, and does not 

 increase from one side to the other as 

 in Mr. Grunow's instrument. 



The two figures, 11 and 12, show the 

 differences between the two camera 

 lucidas at a glance ; / represents the 

 face of the prism in the two cases, c the 

 centre of the field of view on the paper. 

 Fig. II shows the Zeiss instrument, fig. 

 12 the Grunow. It will be understood 



by simple inspection of the two figures, 

 that while the distortion produced by 

 the Zeiss instrument extends outward 

 from the centre of the field of view 

 (fig. 1 1), that of the Grunow increases 

 from one side of the field to the other 

 (fig. 1 2). Such being the facts, a cam- 

 era lucida should be used with great 

 discretion in making drawings for pur- 

 poses of measurement — as, for ex- 

 ample, in drawing blood-corpuscles 

 for microscopical expert testimony. 



I present these facts for what they 

 are worth. 1 do not think the distor- 

 tion produced is of very great conse- 

 quence, in most cases, nor do I see how 

 it can be absolutely eliminated from 

 camera lucidas. It is only when large 

 objects are to be drawn, or, as alrddy 

 intimated, when measurements are to 

 be made, that the distortion deserves 

 serious attention. For my own work 

 it is not worthy of notice. Still, I am 

 surprised to find it so considerable. 

 The microscope should be placed at 

 the proper angle for least distortion 

 at all times, when the camera lucidas 

 is to be used, as these experiments 

 clearly show, and this is best deter- 

 mined by a stage-micrometer in the 

 manner indicated. 



The Visibility of Ruled Lines. 



At a meeting of the Boston Scien- 

 tific Society, held on the evening of 

 Jan. loth. Prof. W. A. Rogers read a 

 paper on fine ruling, a considerable 

 portion of which has already been 

 given in this Journal. A few inter- 

 esting points, however, deserve the 

 attention of microscopists. Prof. 

 Rogers stated that he had ruled 

 bands of lines in which the lines 

 were so fine and delicate that they 

 could not be seen with a microscope, 

 although their spacing was much 

 within the power of the microscope to 

 resolve. Yet he was assured of the 

 existence of the lines. The evidence 

 in support of this assertion was of 

 three kinds : The pressure of the dia- 

 mond upon the glass was sufficient to 



