R. P. WILLIAMS ON CUTTING FRESH FI10ZEN TISSUES. 155 



cold, and covered with felt to prevent the conduction of heat; 

 and the sections are, as I have said, cut by hand. In the form 

 of apparatus before you, the cylinder is fixed in a chamber sur- 

 rounded by the freezing mixture, and the sections are cut by a 

 machine which may or may not be a modification of the American 

 triangular frame instrument incidentally mentioned by Mr. E. T. 

 Newton, in his paper read before the Club last year. Never 

 having seen one of these instruments I made one, but soon found 

 the moving of the three screws to be an intolerable nuisance. I 

 then made another, in which I placed the razor exactly parallel to 

 the base of the triangle, and the setting for thickness I obtained 

 by means of one screw, that at the apex. The screws at the 

 angles of the base are set through, so as to clear the razor-blade 

 of the tissue, all except its cutting edge, when the setting 

 screw is set through as far as it will go. It will be remarked that 

 the value of the setting screw at the cutting edge of the razor is 

 considerably less than in its place at the apex. It is, in fact, in 

 direct proportion to the respective distances of the screw and the 

 cutting edge from the base line, that is to say, as the distance of 

 the setting screw from the base line is to the pitch value of that 

 screw, so is the distance of the razor edge from the base line, to 

 the interval through which it will have been moved by one turn of 

 the setting screw. In the instrument before you one turn of the 

 setting screw sets the razor edge for T i^ of an inch. If, therefore, 

 the head of the setting screw be divided into 10, one of those 

 divisions will set for a section j^ ols inch thick. It may be objected 

 that the cutting angle of the razor is continually being altered by 

 the setting of the screw, but when we remember that within 

 certain limits, where there is no facing plate or guide, the operator 

 cannot be sure at what angle he holds the razor with respect to 

 the tissue which is to be cut, or that he holds it twice at the 

 same angle, this objection will lose much of its force. There is, 

 moreover, a positive advantage when cutting extremely thin sec- 

 tions in holding the razor at a considerable angle. I think it is 

 not sufficiently borne in mind that the last operation of sharpening 

 the razor — taking off the feather edge — practically introduces 

 two new planes ; thus, if we exaggerate the section of a razor,* 



* It will be obvious from an inspection of the figure £P1. xiv., fig. 3) 

 that the angles A, B, C, would have to be ground off before a fine setting 

 could be effected ; or, which would be equivalent, the back of the razor 

 raised, the edge being fixed, until the plane D C is parallel to F P. 



