4:52 lloyal Society. 



discovery in the physiology of the brain ; the first impediment to 

 which, he observes, " is in the nature of the inquiry, since extraor- 

 dinary and contradictory results must be expected from experiment- 

 ing on an organ so fine as that must be which ministers to sensibility 

 and motion, and which is subject to change on every impression con- 

 veyed through the senses." Another cause of fallacy is the depend- 

 ence of the brain on the condition of the circulation within it : but 

 the most frequent source of error is the obscurity which hangs over 

 the whole subject j for although the brain be divided naturally into 

 distinct masses, not one of these grand divisions has yet been distin- 

 guished by its functions -, and hence we may account for the failure 

 of all attempts to explain the phenomena which attend injury of the 

 brain. The principle, now universally admitted, that nerves have 

 distinct functions, and not a common quality, is pursued by the 

 author in his investigation of the structure of the brain, in which he 

 follows the nerves into that organ, and observes the tracts of nervous 

 matter from which they take their origin. He concludes from his in- 

 quiries that both sensibility and motion belong to the cerebrum ; that 

 two columns descend from each hemisphere ; that one of these, the 

 anterior, gives origin to the anterior roots of the spinal nerves, and 

 is dedicated to voluntary motion ; and that the other, which from its 

 internal position is less known, gives origin to the posterior roots of 

 the spinal nerves, and to the sensitive root of the fifth nerve, and 

 is the column for sensation. He further shows that the columns for 

 motion, which come from different sides of the cerebrum, join and 

 decussate in the medulla oblongata j that the columns of sensation 

 also join and decussate in the medulla oblongata ; and lastly, that 

 these anterior and posterior columns bear, in every circumstance, a 

 very close resemblance to one another, in as much as the sensorial ex- 

 pansions of both are widely extended in the hemispheres j for they 

 pass through similar bodies towards the base of the brain, and both 

 concentrate and decussate in the same manner j thus agreeing in 

 every respect, except in the nervous filaments to which they give 

 origin. Hence he explains the phenomena of the loss of sensibility 

 as well as the power of motion of one side of the body, consequent 

 on injuries of the other side of the brain. 



The Society then adjourned over Whitsun Week to the 29 th of May. 



May 29. — A paper was read, entitled, " On the Principle of Con- 

 struction and General Application of the Negative Achromatic Lens 

 to Telescopes and Eyepieces of every description." By Peter Barlow, 

 Esq., F.R.S. 



This paper is intended as a more full illustration of the principles 

 on which the negative achromatic lens is constructed and applied, than 

 has been given in the extract from the author's letter to Mr. Dollond, 

 contained in the paper of the latter, lately read to the Society, on his 

 ingenious application of that lens to the micrometer eyepiece*. The 

 author shows that its advantages are not confined to this instrument, 



* An abstract of Mr. Dollond's paper appeared in Lond. and Edinb. Phil. 

 Mag., vol. iv. p. 364. 



