132 



direction, it becomes the most obvious means of marking the direc- 

 tion so exhibited throughout a finite extent of line. The series of 

 directions transverse to a given normal may then be represented by 

 two straight lines crossing each other at right angles, and an inde- 

 finite number of other straight lines diverging from the point of 

 intersection, and dividing the plane surface round that point into as 

 many parts as there are diverging lines. If now we take two of 

 these lines, like the hands of a clock, and suppose one to remain 

 fixed while the other revolves from left to right, it will pass success- 

 ively through all the directions intermediate between left and front, 

 while the quantity of plane surface intercepted between the hands 

 abutting on the point of intersection will continually increase as the 

 difference in their direction becomes greater, or in proportion as 

 distance in the direction of the moveable hand contains a greater 

 proportion of distance in the direction transverse to that of the fixed 

 one. Thus we are taught a new mode of estimating the relation 

 between the direction of straight lines diverging from a common 

 point ; not by a proportion which addresses itself to the understand- 

 ing merely, but by a quantity admitting of measurement by bodily 

 comparison, viz. by the quantity of plane surface intercepted between 

 the diverging lines and abutting on the point of intersection, or by 

 the magnitude of the included angle. 



Professor Challis gave an account of a luminous appearance ob- 

 served at the time of the perihelion passage of Klinkerfue's comet. 



Professor Stokes read a paper on the Optical properties of Light 

 reflected from Crystals of Permanganate of Potash. The substance 

 of this paper is embodied in a paper on the Metallic Reflexion exhi- 

 bited by certain Non-metallic Bodies, published in the Philosophical 

 Magazine, vol. vi. p. 393. 



December 12, 1853. 



Professor Fisher read the first part of a paper, entitled " Researches, 

 Physiological and Pathological, on the Development of the Vertebral 

 System." 



After having explained what he meant by the term vertebral 

 system, he stated (and he illustrated what he described by drawings) 

 that the spinal marrow, at a particular stage of growth of the human 

 embryon, exhibits indications of segmental development correspond- 

 ing to that of the spinal column ; that is to say, that each of its 

 halves offers on its external surface a series of symmetrical spaces 

 defined by transverse lines, each of which spaces corresponds to the 

 roots of a single spinal nerve ; and again, that each half presents in 

 its internal structure, a double series, one anterior, the other poste- 

 rior, of symmetrical areas, two of which appeared to equal in extent 

 one of the external spaces just spoken of. Professor Fisher also 



