39 



On a New Camera Lucida. 



By Edward M. Nelson, F.R.M.S. 



{Bead November 16^, 1894.) 



The well-known neutral tint of Dr. Beale is such a simple 

 and inexpensive form of camera that it seemed a pity that the 

 only drawback to its coming into more general use, viz., that 

 of transposing its erect image, should not be corrected. This 

 drawback is a serious one, because a picture drawn by a Beale's 

 camera only becomes similar to the original object when it is 

 viewed as a transparency from the wrong side of the paper. 

 For instance it is well known that some insects have one leg 

 on one side of their bodies different to the corresponding leg on 

 the other side, and it is necessarily important that such micro- 

 scopic objects should be depicted correctly. Now all we have 

 to do is to correct the transposition without altering the image 

 in any other manner. Obviously this can be accomplished by 

 adding a lateral reflection. If, therefore, we place over the 

 eye-piece, at an angle of 45°, a small silvered mirror (a first 

 surface mirror is unnecessary, a piece of ordinary silvered 

 glass, such as is used in sextants, answers every purpose), so 

 that when the microscope is placed in a horizontal position the 

 image may be reflected at right angles to the body, either to 

 the right or left hand of the microscope in a plane parallel to 

 that of the table, and then if we intercept this horizontal beam 

 by an ordinary Beale's neutral tint, an erect image, with its 

 transposition corrected, will be reflected upwards to the eye, 

 and seen on the table through the neutral tint in the usual way. 

 In brief, the lateral reflection corrects the transposition, w^hile 

 the vertical reflection forms the first surface of the neutral tint, 

 the inversion of the image. The resultant image is, therefore, 

 precisely similar to the object on the stage of the microscope. 

 Anyone possessing a right-angled prism can produce the same 

 effect by placing it anywhere between the objective and the 

 eye-piece, and by placing an ordinary neutral tint on the eye- 

 piece ; of course, the horizontal position of the right-angled 

 body, when the microscope itself is in a horizontal position, 

 must be maintained. 



