CORRESPONDENCE. 47 



force. The eye-piece is expensive. Not only is it, at least so far as 

 regards all below the prisms, perfectly achromatic, but of very peculiar 

 and perfect construction, and as to liability of derangement, I can 

 safely say it is as firm as any binocular arrangement now known, 

 and easily adjusted if it should become deranged. I am told that 

 M. Hartnack has made a similar eye-piece. During the Exposition 

 I placed Mr, Tolles' eye-piece in his hands for inspection, and it 

 remained with him for several days. I cannot say whether he copied 

 it, or whether his arrangement is different in princij)le, or the same, 

 as I have not seen it. M. Hartnack appeared to me to be far too 

 honourable a man, as MM. Nachet, to whom also I showed it, most 

 certainly were, to take any credit for an invention truly belonging 

 to another. 



H. L. Smith, 



Formerly of Kenyan College. 



The French Erecting Prism a Camera Luoida. 



To the Editor of the ' Monthly Microscopical Journal' 



Reading, June 5th, 1871. 



Sir, — Needing greater amplification for minute dissections and 

 diatom selection than I could obtain from the simple lens of the 

 dissecting microscope, and unable to overcome life-long habit and to 

 reverse every movement which the inverted position of the object 

 under the compound instrument necessitates, I applied to Mr. Curties 

 for an erector, who furnished me with a French eye-piece erecting 

 prism, which effects my object very completely. 



I find, however, that it subserves another pui'pose, for which cer- 

 tainly it was never originally designed, and, so far as I know, has 

 hitherto remained unsuspected — that of an effective camera lucida, 

 with which very satisfactory outlines can be made, either in the ver- 

 tical position of the microscope, on a screen placed behind it, or in 

 the ordinary horizontal one (by far the most convenient in practice), 

 on a paper extended below it on the table, the usual focal distance of 

 ten inches being in both cases maintained. Prohatum est ! But how 

 is the image produced and the sketch obtained? Backed by metal 

 plates, with the image of the object on the stage refracted by the prism 

 through the small circular opening of the front plate, on to the retina 

 of the observing eye alone, it is obvious that the image seen on the 

 screen or on the table cannot he due to any reflexion. It must, therefore, 

 as suggested by my friend and neighbour, Dr. Shettle, be illusion only 

 — a spectre, a brain phantom — though definite and capable of delinea- 

 tion. Have we not here a clear demonstration of the physiological 

 fact, " that though the image of an object be impressed on the retina 

 of one eye exclusicely, through the decussating nerve fibres of the optic 

 commisure, it makes an equal visual impression on that of the other"? 

 It may be objected, I know, that the one eye sees the image, the other 

 the point of the pencil only; that the one records what the other 



VOL. VI. E 



