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XVI. — On a Neiv Camera Lucida. By Dr. Hugo Schroder. 



CRead 10th October, 1883.) 



In the recent volumes of the Journal of this Society I have met 

 with descriptions and figures of several forms of camera lucida 

 which were new to me. I obtained an example of each, and made 

 a series of trials in comparison with the older forms with which I 

 was abeady familiar. In all of them I found more or less defects, 

 such as limitation of field, distortion, indistinctness of image or of 

 drawing-point, awkwardness of position, &c. Being engaged later 

 in endeavouring to simplify and perfect the construction and adjust- 

 ment of Mr. Wenham's high-power binocular prism, I was much 

 interested by the ingenuity of this device, and it occurred to me 

 that that arrangement of prisms might be modified, so as to be 

 available as a camera lucida in which the defects of the forms 

 hitherto made would be considerably reduced if not entirely 

 eliminated. 



Assuming a 45° inclination of the Microscope to be the position 

 most generally convenient for drawing, I (in June last) drew on 

 a large scale the system of prisms which appeared to me suitable 

 for a camera lucida. Messrs. Boss undertook to construct the prisms 

 to my drawings, and the apparatus was found upon trial to answer 

 my expectations fully. I am induced to describe it here because it 

 has also met with much approbation from microscopists, who were 

 previously disinclined to believe in the possibility of any new 

 device at the present day, which should be substantially better 

 than the numerous older forms which apparently exhausted the 

 subject ! 



It is well known that all forms of reflecting prisms acting by 

 means of one reflection are extremely sensitive in regard to the 

 position of the mirror in relation to the Microscope, as also in a 

 less degree in relation to the eye ; the slightest deviation from the 

 normal position in many cases entirely destroying the effectiveness 

 of the apparatus. For this reason camerse lucidae acting by one 

 reflection have not found favour, though their apparent simplicity 

 has induced the construction of many such forms. 



In order to obviate the difficulties incident to the use of one 

 reflection, many devices have been made acting by two reflections, and 

 where these have been so contrived as to act like parallel mirrors 

 the reflected image has possessed the advantage peculiar to this 

 principle, of being practically insensitive to slight differences of 

 position relative to the Microscope or to the eye, remaining in fact 

 stationary within a considerable range of adjustment, as in 

 Wollaston's camera lucida. 



