312 



NATURE 



{July 1 8, 1878 



A NEW CAMERA LUCID A 



THE various kinds of camera lucida hitherto used 

 have always possessed many inconveniences, none 

 of them allowing to be seen upon the paper with suf- 

 ficient precision, and simultaneously, the image of 

 the object and the point of the pencil. For the purpose 



of remedying this inconvenience. Dr. J. G. Hofmann, of 

 the Rue Bertrand, Paris, has had recourse to an arrange- 

 ment by which he believes he has obtained the most 

 satisfactory results. The illustration will give some idea 

 of this arrangement. 



^g- I i"epresents the general elevation, in half size, 

 of Hofmann' s camera lucida. Fig. 2 is a transverse 



section of the optical part, composed, at A, of a metallised 

 mirror, or other metallic surface, polished and rigorously 

 plane ; at B, of a small plane mirror of parallel glass, 

 forming, with the metallised mirror, a fixed angle. The 

 function of the latter is to let pass a part of the luminous 

 rays coming from the object to be drawn, and to show at 

 the same time the point of the pencil alongside the image 

 upon the paper. At G may be placed, in a movable 

 frame, either a plate with parallel surfaces, or lenses of 

 neutral glass of various foci, the principal object of which 

 is to enable a satisfactory drawing to be made of the 

 objects placed inside, when using white paper ; for the 

 outside, this glass serves to temper the brightness of the 

 sun. 



At C is the eye-hole or opening before which the eye is 

 placed. The knob D serves to place the chamber in a 

 convenient position, which sometimes depends on that of 

 the artist with respect to the object ; but generally it is 

 convenient to place the mirror D vertically. With the 

 same pieces of the optical part, with the addition of a 

 concentrating lens. Dr. Hofmann has been able to con- 

 struct a second model applicable to microscopes, for 

 which, as well as for telescopes, all previous forms of 

 camera have given only very mediocre results. 



ANA TOMICAL PREP A RA TIONS FOR MUSEUM 

 AND CLASS PURPOSES 



T N a former number of Nature (vol. xvi. p. 360) I offered 

 ■'■ some suggestions on museum preparations and 

 arrangement. These I can now supplement by a new 

 method which I have tried with encouraging success. 



No museum-curator needs to be reminded of the many 

 defects of the ordinary fluid-preservatives. Evaporation, 

 blanching, spilling, optical distortion, the cost and incon- 

 venient shape of glass vessels — these are among the 

 serious and apparently inevitable advantages of dilute 

 spirit. I have found it possible to get rid of all these 

 difficulties together by mounting dissections and entire 

 animals in glycerine jelly. 



The following directions may be followed until expe- 

 rience shall suggest better. Soak gelatine (best quality) 

 in water until it has absorbed as much as it can, melt 

 and add an equal bulk of best German glycerine. 

 Clarify with white of tgg, one egg to a pint of mixture, 

 taking care to boil very steadily, without burning. Filter 

 hot through flannel. The jelly should be transparent, 

 and of a pale straw-colour. It should melt at 39° C., and 

 have a specific gravity of i*i86 at 8° C, compared with 

 water at the same temperature. 



The jelly may be diluted with water, with glycerine, or 

 with a mixture of the two. I find one part of jelly to one 

 of glycerine and one of water a convenient proportion. 

 The dilute jelly is apt to run fluid on exposure to the air, 

 owing to the growth of moulds. This may be prevented 

 by using a solution of salicylic acid or thymol in water for 

 dilution. These substances cause opalescence in the 

 medium, but a very minute quantity of acetic acid clears 

 it again. 



Lay out the dissection on wax, as recommended in my 

 previous letter, but without pins, and fill up with jelly 

 rendered fluid by gentle heat. When the vessel is full, 

 allow the jelly to cool and set, then pour a httle more on 

 the top. After this also has set, lay the glass cover 

 (warmed by immersion in hot water) in its place. As the 

 superficial layer of jelly melts, press the cover down. 

 When cold, cement the edges with strips of cloth smeared 

 with coaguline. 



The vessel for mounting may be of almost any size and 

 shape. I have tried glass jars, built-up glass cells, 

 ebonite, gutta percha, earthenware, and wood soaked in 

 paraffin. The vessel should be strong and quite air-tight. 



It is early as yet to speak of the final result. Some 

 preparations have lasted five months without alteration. 



