of British Opt inn. 73 



them to penetrate to an equal depth with a single one, and 

 consequently, will be less easily managed, and with more dif- 

 ficulty be prevented from penetrating too deep. But however 

 equally sharp they may be when first used, they will continue 

 so but a very little time. A small diflference in the thickness of the 

 blades, in the material of which they are made, or of their 

 temper, will prevent this. They will then require unequal 

 pressure on their several blades, and, therefore, will become still 

 more unmanageable. The inventors of these double instru- 

 ments have either foreseen this consequence, or have afterwards 

 become sensible of it from experience ; and have, therefore, 

 contrived gauges, or guards, to be added to them, to prevent 

 the blades from penetrating further than to a certain depth. 

 Without some contrivance of this kind, it is evident that no 

 double instrument can well be used in the operation in ques- 

 tion. And these guards, it is apprehended, can only have their 

 proper effect when used on a plane, or at least a regularly 

 bending surface. But it is allowed *, that the capsules of the 

 white poppy are irregular and uneven; and, consequently, the 

 incisions of the guarded blades, when applied to them, will be 

 unequal in their depth. Add to this, that these double instru- 

 ments, with their several appendages, cannot but be compli- 

 cated, unwieldly, and expensive. 



Again — The best single blade that I have yet been in pos- 

 session of, has required, when in full work, to have its edge 

 sharpened once a day. That the necessity of doing this may 

 not, at any time, interrupt the work Mfhen circumstances are 

 favourable to it, it is expedient that each person employed in 

 my method of operating, should have the pocket which has 

 been described, as making part of the apparatus, furnished 

 with a spare lancet-blade or two, fit for immediate use, as like- 

 wise a spare quill or two. With this provision, the operator 

 may take the advantage of a leisure half-hour, occasioned by a 

 shower of rain, or the intemperance of the sun's heat, or in the 

 evening, to sharpen his instrument. But what is to be done 



* See Edinburgli Philosophical Journal, No. 3, p. 262. 



