22 Transactions of the Society. 



(Fig. '6), containing a reflector at N, is employed, the camera fitting 

 over it at Gr, and the whole being inserted into the tube of the 

 Microscope by the part H. 



The instrument is thus suitable for powers up to 500 ; beyond 

 this limit, however, it is desirable to substitute for the colourless 

 glass plate B a tinted one. 



The camera, to use Dr. Hofmann's expression, " suppresses all 

 existing eye-pieces," but with objects requiring only small mag- 

 nification to be within the field of the camera the arrangement is 

 employed which is shown in Fig. 4. It consists of two plano- 

 convex lenses of difierent foci, and slides into H. 



The part No. 2 may be used alone. No. 3 being taken away. 

 If the image of the object is still beyond the field of the instrument, 

 the lens in No. 2 is unscrewed, and No. 3 replaced, which gives a 

 second amplification ; and with both lenses in their place a third 

 is obtained. 



Dr. Hofmann writes that this apiiaratus is the result of an 

 exjoenditure of no little time and thought on his part, and that it 

 has been very highly commended by leading men on the Continent. 



(2) A second form also originates in France, and is the invention 

 of M. Pellerin, who describes its principle in the ' Comptes Kendus ' * 

 of the French Academy. 



With the view, as he expresses it, of avoiding the weakening of 

 one of the images through reflection by a transparent plate as in some 

 forms, and the irksomeness of others which require that the object and 

 the drawing should each be viewed with half the pupil, he suggests 

 the following arrangement, which is an imitation of M. Cornu's 

 polarizer, and gives two images of the same intensity and visible at 

 the same time by the whole of the pupil. 



A Wollaston camera lucida being made of glass having an index 

 higher than the extraordinary index of spar, there are joined to the 

 face which has an angle of 135° a plate of spar and a prism made 

 of the same material as the camera, having its second face parallel 

 to the face whence the rays emerge. Thus, at a suitable inclination, 

 one-half the light coming from the object will be totally reflected as 

 extraordinary rays, and a part of the light coming from the drawing 

 will be transmitted as ordinary rays. The portions reflected and 

 transmitted will be each one-half if there is no reflection of the 

 ordinary rays, the condition for which is, that the glass of the two 

 prisms and the cement which unites the pieces shall have the ordi- 

 nary index, and in practice this can always be approximately 

 attained. 



For these assumed conditions, and the plate of spar being per- 

 pendicular to its axis, the following calculation is given of the field, 

 which is then equal in all directions : in the interior of the glass 



Vol. Ixxxvi. p. 764. 



