436 PHILOSOPHICAL TRANSACTIONS. [ANNO \7QQ. 



To see how far this structure was peculiar to the optic nerve, similar experiments 

 were made on the internal substance of the 5th and 7th pair of nerves, near their 

 origin at the brain, and the structure was found to be the same. In these last 

 mentioned nerves, the interstices between the fasciculi were smaller than in the 

 optic nerve, rendering their transverse sections less transparent; from which it is 

 natural to suppose, that the internal parts of the optic nerve are not so compact as 

 in other nerves, and therefore it is better fitted for examination. 



These experiments show, that the nerves do not consist of tubes conveying a 

 fluid, but of fibres of a peculiar kind, different from every thing else in the body, 

 with which we are acquainted. The course of these fibres is very curious; they 

 appear to be constantly passing from one fasciculus to another, so as to connect all 

 the different fasciculi together by a mixture of fibres. This is different from the 

 course of blood-vessels, lymphatics, or muscular fibres: the only thing similar to 

 it, is in the formation of nervous plexuses; which leads to the idea of its answering 

 an essential purpose, respecting the functions of the nerves. 



J I. Observations on an Unusual Horizontal Refraction of the Air ; with Remarks 

 on the Variations to which the Lower Parts of the Atmosphere are sometimes Sub- 

 ject. By the Rev. S. Vince, A.M., F.R.S. Being the Bakerian Lecture, p. 13. 



The uncertainty of the refraction of the air near the horizon has long been 

 known to astronomers, the mean refraction varying by quantities which cannot be 

 accounted for from the variations of the barometer and thermometer; on which ac- 

 count, altitudes of the heavenly bodies, which are not more than 5° or 6°, ought 

 never to be made use of when any consequences are to be deduced from them. 

 The cause of this uncertainty is probably the great quantities of gross vapours, 

 and exhalations of various kinds, which are suspended in the air near the earth's 

 surface, and the variations to which they are subject; causes of which we have no 

 instruments to measure the effects they produce, in refracting the rays of light, 

 In general, the course of a ray passing through the atmosphere, is that of a curve 

 which is concave towards the earth, the effect of which is to give an apparent ele- 

 vation to the object; and thus the heavenly bodies appear above the horizon, when 

 they are actually below it; but it will not alter the position of their parts, in respect 

 to the horizon, that is, the image of the highest part of the object will be upper- 

 most, and the image of the lowest part will be undermost. The figures however 

 of the sun and moon, when near the horizon, will suffer a change, in consequence 

 of the refraction of the under limb being greater than that of the upper; from 

 which they assume an elliptical form, the minor axis of which is perpendicular to 

 the horizon, and the major axis parallel to it. But a perpendicular object, situated 

 on the surface of the earth, will not have its length altered by refraction, the 

 refraction of the bottom being the same as that of the top. These are the effects 

 which are produced on bodies at or near the horizon, in the common state of the 

 atmosphere, by what I shall call the usual refraction. 



