138 MEMORANDA. 



difficulty in using the camera lucida as at present constructed, 

 owing to the constrained position in which the eye must be 

 held, half the pupil over and half beyond the edge of the 

 prism. A partial remedy for this difficulty would be found in 

 discarding the present form of prism, with its two reflecting 

 surfaces, and using a prism having only one such surface, and 

 drilling a small hole through it vertically. Through this hole 

 the paper would be seen, while the image would be visible by 

 the rays reflected from the inclined surface of the prism. 

 The objection to this is that the hole would act, as regards 

 the rays entering the prism, as an opaque rod, and so render 

 useless the portion of the reflecting surface immediately 

 behind it. A complete remedy for the difficulty is suggested 

 by inspection of Dr. Carpenter's figure. Instead of making 

 a hole in the prism, let there be attached to the centre of 

 its inclined surface, by Canada balsam, an oblique segment 

 of a small glass cylinder,* so that its base should be pa- 

 rallel to the upper surface of the prism. The effect now, 

 on looking into the prism, will be precisely that of a 

 a hole through it, without the drawback attendant upon an 

 actual hole. The paper will be clearly seen through the 

 prism and the cylinder, and the image by reflection from the 

 inclined surface of the prism, the whole of which surface will 

 now be available, with the exception of the spot where the 

 cvlindrical segment is attached, which, however, will be so 

 small as not to be productive of any injurious effect. In fine, 

 so far as I at present see, I feel warranted in expressing a 

 belief that by the adoption of the arrangement now suggested, 

 the difficulty hitherto attendant on the use of the camera 

 lucida would be entirely prevented. 



It has often been matter of wonder with me why our 

 opticians continue to supply, for microscopical purposes, the 

 prism with two reflecting surfaces. These ai'e requisite in 

 other applications of the camera, for the erection of the 

 image. But in its application to the microscope we do not 

 want this.f What we want, if we had a preference in the 

 matter, is that the inversion caused by the first reflection be 



* Dr. Carpenter calls Nachet's "piece e" a prism. 1 think lie musi he 

 wrong. The quasi hole will be of the form of a direct section of the piece 

 employed. A square prism would give a square hole, ami a cylinder a cir- 

 cular one. 



f A polished steel disc (Amici's disc) has sometimes been employed 

 instead of the more usual Wollaston's camera. But the latter will always 



he preferred bj those who draw from the microscope, simply I'm- the reason 

 that the image thrown on the paper by it corresponds in position with that 

 viewed through the microscope. — [Eds.] 



