CUTTING SECTIONS OF THE EYES OF INSECTS, &C. 9 



simultaneously revolved three times, being equivalent to a straight 

 draw of three inches for every -J^. of an inch ; thus, if the head or 

 object be T ^ of an inch thick, and a cut be made through it, the 

 cutter will revolve six times, or eighteen inches of cutting edge 

 will have been employed. I would point out, too, another advan- 

 tage, in the fact that the motions of the object and cutter are cor- 

 related, so that if the motion of the object is retarded, or stops 

 altogether, so does also that of the cutter. I have tried the effect 

 of great speed, by means of a multiplying wheel, but was so ill 

 satisfied with the result, as to return with greater confidence to the 

 velocity here adopted; there appears to be a very sensible advantage 

 in a tolerably quick, very steady, and correlated speed. It (the 

 instrument) cuts with a mechanical ease — so to speak — which de- 

 lights me every time I use it. 



A clear idea of the construction of this machine may be obtained, 

 if you conceive of it as being composed of two parts, one having 

 two dove-tail slides urged by screws, at right angles to each other, 

 like the slide rest of a lathe, for carrying the object underneath the 

 cutter, and for adjustment. The other part is a contrivance for 

 transmitting while changing the direction of the motion of its 

 traversing screw to the cutter. It is, you will see, then, not quite 

 60 complex as it might at first sight appear to be, and is suitable for 

 cutting any soft tissue. One word with reference to the shape of the 

 cutter. It is of the utmost importance with this cutter, as with 

 any other, that it should be perfectly flat at the extreme edge, 

 even the feather edge thrown up in the process of sharpening, must 

 be removed by attrition with an equally flat surface. 



While recommending thus briefly to your notice a machine 

 which has cost me many troublesome, not to say anxious moments, 

 I am vividly conscious that no machine, however delicately perfect, 

 can supply the place of that happy knack of doing things which 

 is such an essential element of success in microscopical manipula- 

 tion. But there can be no real objection to the proper use of 

 1 instruments for all that, for I make bold to say, that, although 

 there may be members present who can cut a very presentable 

 section with an ordinary pen-knife sharpened for the purpose, yet, 

 where great delicacy is required, the ordinary means must perforce 

 be superseded by that which promises better results, and is 

 therefore of more delicate construction. 



It will be seen that I have drawn no comparison between this 



