The Apparent Immobility of Spectral Impressions, (Sfc. 147 



2. The Composite Structure of the Bipyramidal Sulphate of Potash not 

 discovered by Mr Brooke. 



Our mineralogical readers are, no doubt, aware of the bipyramidal form 

 in which sulphate of potash often crystallizes. Count Bournon considered 

 this as the primitive form of the salt. In a paper in the Annals of Phi- 

 losophy, Mr Brooke has described this form of the salt, and shows that it 

 is a composite form, consisting of rhomboidal prisms, combined in the 

 manner which he has represented in a diagram. 



This composite form of the bipyramidal dodecahedron of sulphate of 

 potash had been discovered long before by the agency of polarised light, 

 and the combination distinctly described in the first paper of the first num- 

 ber of the Edinburgh PhilosophicalJournal, a work to which Mr Brooke was 

 a contributor. As Mr Brooke has made no reference whatever to that pa- 

 per, it might have been presumed that he had not read it ; but we find that 

 he has actually read and quoted it in his lucubrations on the structure of 

 Apophyllite, with which he has favoured the public; and which have al- 

 ready shared the same fate as his rhombohedral speculations on the primi- 

 tive form of the sulpha to- tri- carbonate of lead. 



3. The apparent immobility of Spectral Impressions ; their Singleness 

 by Distorted Vision ; and the Reference of the Phenomena of Vision to Vo- 

 luntary Muscular Action, first discovered and proposed by Dr Wells, and 

 not by Mr Charles Bell. 



In the observations which were some time ago made on spectral impres- 

 sions in this Journal, the author conceived, from reading Mr Charles Bell's 

 paper, that that gentleman had the sole merit of discovering the apparent 

 immobility of ocular spectra, and the fact of their remaining single by dis- 

 torted vision ; and that he had been the first who referred these and other 

 phenomena of vision to the voluntary actions of the muscles. This opinion 

 was founded on the circumstance of Mr Bell not having mentioned any 

 other author as having preceded him in these views ; and upon looking 

 again at Mr Bell's paper, it is obvious that every reader must consider him 

 as the discoverer of these facts, and the author of these views. We find, 

 however, that these facts and views are all contained in Dr Wells' Essay 

 upon Single Vision with Two Eyes, Lontl. 1792, p. 65,66, 67, 70; and 

 are stated with a degree of philosophical precision very different from 

 that which they wear in their revived form. Our inference from these facts is, 

 that Mr Bell was not acquainted with the work of Dr Wells, otherwise he 

 would not have failed to do justice to the previous labours of that able and 

 ingenious philosopher. In a future number we shall discuss this subject 

 with Dr Wells, and give a full account of the experiments we have made 

 upon it. Having overthrown the doctrine of the involuntary motion of 

 the eye-ball by optical arguments, we shall leave the physiology of that 

 part of the subject to Dr Knox, who, in a paper lately read before the 

 Royal Society of Edinburgh, has demonstrated the incorrectness of Mr 

 Bell's reasonings, and the fallacy of his results. 



