Messrs. Cross and Bevan on Pseudo-Carbons. 325 



will be seen to coalesce; and the attention can then be trans- 

 ferred to their resultant combination. This is a hemispherical 

 lustrous cup, if the stereograph be that of the moon, which is 

 smaller and nearer than that obtained by use of the prisms. By- 

 pulling the cross bar nearer, the cup becomes smaller, relatively 

 shallower, and, for obvious mathematical reasons, elliptic with 

 the minor axis horizontal. The wooden screen can now be 

 folded down, as before, and attention be given to the variable 

 size of the image as determined by the conditions under which 

 it is seen. 



The semi-lenses may be restored, and the screens arranged 

 as in fig. 6. The cross bar, a, is removed; and in its place is 

 put one upon which are pivoted a pair of revolving bars, 

 whose "springs hold in position a pair of cards on which are 

 perfectly similar outline figures, such as series of concentric 

 circles. The production of stereoscopic effects, either ortho- 

 scopic or pseudoscopic, at will, by oblique projection upon the 

 curved surfaces of the retinas, has been already fully discussed* 

 and now needs no explanation. 



40 W. 40th St., New York, 

 February 27, 1882. 



XXXYIII. OnPseudo-CarUm. 

 By C. F. Ckoss, B.Sc, and E. J. Beva^j. 



npHEKE is a general knowledge amongst chemists that the 

 JL term " carbon " has been loosely applied to a number of 

 substances whose composition and properties, on the experi- 

 mental side, and the consideration of the condition of whose 

 formation, on the theoretical side, completely prove their non- 

 elementaiy nature. Notwithstanding this, a glance at the 

 most modern text-books will show that coal is still spoken of 

 as " a form of carbon ;" charcoal also ; and the action of sul- 

 phuric acid on sugar is still taught to consist in the resolution 

 of the carbohydrate into carbon and water?, the dehydrating 

 energy of sulphuric acid being the efficient cause. It is ne- 

 cessary therefore that these defects in our terminology and 

 classification should be stated, in order that thev may be 

 removed. In reviewing this subject we would first account 

 for the existence of the errors in question. Certainly the cause 

 would seem to lie in the inertia of the chemical mind — a desire 

 to know without ascertaining, a tendency to accept a simple 



* See the preceding article. 

 t Communicated by the Authors. 



X This explanation "may be dismissed without discussion. 

 Phil. May. S. 5. Vol. 13. No. 82. May 1882. 2 C 



