338 Dr. Forster on the dispersive Poser of the Atmosphere. 



R S. I should like much to know the truth of an opinion I 

 have heard broached, that Dr. Bradley deduced his tables of 



error is owing to an actual change in the brain, either from nervous dis- 

 order and from increasing age, or, as I think may happen in some instances, 

 from a change of activity from one to the other hemisphere of the brain. 

 All the cerebral organs are double, and the organ of time among the rest. 

 New, if the action of the hemispheres respectively does not exactly cor- 

 respond, we may conceive an error to be invariably produced, by the left 

 hemisphere, For example, taking the duty long performed, by the right 

 hemisphere. For physiologists have shown strong reasons for thinking 

 that the double organs do not both act at once ; but alternate at long 

 periods of time and thus relieve one another. Of course this explanation 

 is conjectural, and only founded on analogies that none but phrenologists 

 can readily enter into. Nevertheless, the discussion of these subjects is the 

 best way to arrive at truth. And astronomers will excuse me for intro- 

 ducing a subject not properly belonging to that science; since it tends to 

 show the cause of the error complained of, and since the healthy condi- 

 tion of our organs of sense and the brain is quite as necessary as is the per- 

 fection of optical instruments to correct observation. 



I may mention here another curious fact, — that if certain persons accus- 

 tomed to observe with the right eye, begin to use the left, they at first are 

 liable to put down an excess + of altitude. This has been explained, 

 though unsatisfactorily, as follows : — The two eyes are seldom so exactly 

 alike as that they shall both give the object viewed the same elevation ; 

 although from habit when both eyes are open we see but one object. Now 

 it may appear at first view, that this circumstance would not produce an 

 error, inasmuch as all objects being elevated or depressed alike by each eye, 

 the altitude of a star viewed with the left eye would appear at the same 

 relative distance from the horizon, as when viewed with the right eye, with 

 which we had been accustomed to view it. This seems good reasoning at 

 first, but the fallacy of it consists in this : that though when we change the 

 use of the more to that of the less elevating eye, the distances of all the 

 objects bear the same relative proportion to each other, yet no particular 

 object appears at the same relative distance, as it before did, from the 

 place where we conceive the horizon or any other terrestrial objects to be, 

 6y the sense of touch. And this circumstance, unperceived by ourselves, 

 creates a sort of confusion in the mind, that perplexes one in the record of 

 small quantities, so as to produce error. The surplus of elevation given by 

 one eye over and above that of the other, whatever inequality exists, may 

 be measured, in cases where we can induce temporary double vision, or by 

 looking alternately with each eye, or by looking through a telescope with 

 one eye, and at the same object with the other eye naked. But the dis- 

 cordance varies with the particular inclination of the eyes. Now I am dis- 

 satisfied with the above explanation, because the use" of the micrometer 

 would prevent any such error from such a cause : and I am inclined to 

 refer it to the parts of the brain in connexion with the eyes, called by the 

 physiologists organs of space and size. For if those organs in one hemi- 



Shere did not correspond in action to those in the other, a change to 

 e use of the other side might cause the observer actually to perceive 

 differently the quantities of space. The organs of the left side, for exam- 

 ple, might not perceive such small quantities of space as those on the 

 right. All this is very obscure and hypothetical at present, and we must 

 be very careful not to perplex our observations of phenomena, bv obtru- 

 ding too hastily any theoretical explanations of their physical causes. 



refraction. 



