ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 567 



contiguous spaces were measured, showing an increase from the 

 centre outward of 3 diameters. 



The above figures should not be considered as absolutely accurate, 

 but approximately correct — sufficiently so to illustrate the subject. 



The Grunow instrument shows the entire field of view on the 

 paper. Measuring from the middle of the surface of the prism to the 

 margin 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 centre beiag 17° from that margin, and 14° from 

 the opposite side. Hence, the distortion produced diminishes slightly 

 toward the centre from either side ; but the real difference from side 

 to side is shown by the above 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 

 Grunow's instrument. Such being the facts, a camera lucida should, 

 the author thinks, be used with great discretion in making drawings 

 for purposes of measurement — as, for example, in drawing blood- 

 corpuscles for microscopical expert testimony. He does not think, 

 however, the distortion produced is of very great consequence in 

 most cases. It is only when large objects are to be drawn or 

 measurements to be made that it deserves serious attention. 



Measurement of Microscopical Magnitudes. — The article which 

 Professor Melassez refers to above is contained in an earlier portion 

 of the same volume,* in which he states his view that the different 

 processes for measuring the linear magnification of objects seen under 

 the Microscope give only approximative and inexact results, and after 

 describing briefly the two methods generally employed, viz. the 

 micrometer eye-piece and the camera lucida, he proceeds to consider 

 the causes of the want of exactness proper to the latter, and explains 

 how, by modifying it slightly, its errors may be corrected. 



In the following diagram (fig. 103) A B is the object f of unknown 

 dimensions on the stage of the Microscope, C D the camera lucida, O the 

 optic centre of the eye. Unite by lines the point O to each of the 

 extremities of the object A B, and draw the visual rays which, starting 

 from the point O, follow at first the direction of the rays A and 

 O B, but which, instead of going through the camera, are reflected at 

 C and at D, and then pass out of it. We know that by interposing 

 a sheet of paper in the path of these rays we are able to see and to 

 draw on the paper the image of the object A B. a& is the drawing 

 obtained at a distance from the eye equal to that of the eye from the 

 object ; a' b' that obtained at the distance of distinct vision ; a" b" that 

 obtained on the table on which the Microscope stands. As is shown 

 in the figure, the drawings will be the larger as the paper is further 

 from the eye, although the image in all these positions always appears 

 of the same size to the person drawing, which is due to the fact that 



* Ranvier's Travaux, p. 114. 



t Strictly speaking, A B is tlie image of the object. 



