698 Transactions of the Society. 



To reply to the second question ; — Most assuredly no special 

 knowledge of drawing is needed for making accurate outlines 

 with the aid of a prism ; little more than the first lesson in free- 

 hand drawing is required, viz. the power of tracing lines with 

 firmness and certainty of touch. 



The third question, as to the best form of prism, will he met 

 by a short review of the various forms of drawing appliances from 

 the days of WoUaston, who devised a prism which in many of its 

 qualities has never been surpassed. In making such a review this 

 evening, as I shall have to " name names," it need hardly be said 

 I shall strive to keep within the bounds of fair criticism, and 

 especially to eschew invidious comparisons. Time would not allow 

 me to go into the optical construction of the various appliances 

 which I shall have to bring before your notice ; all these particulars 

 can be learned from the back numbers of our Journal. I prefer to 

 take these little adjuncts to the Microscope just as they were sup- 

 plied to me by their more or less sanguine inventors, and to narrate 

 what they have respectively done in my hands, premising a hope 

 that as I have had one or the other in pretty constant use for some 

 forty years, a description of their performances will neither be 

 unwelcome nor unprofitable to the practical microscopist. 



In illustration of the remarks I have to make, and as showing 

 the various applications and " all round " character of the drawing 

 prism — and particularly in its more recent forms, I venture to 

 exhibit selections from among the thousands of drawings I have 

 made, choosing those which may be said to be typical of the uses 

 of the prism. 



The subjects are, as you will see, of all sorts, but having this in 

 common, that they were all drawn under the Microscope ; all out- 

 lined by the prism. When you see copies from photographs, from 

 book-illustrations, magnifications of the exquisite engravings in 

 Tarrell's 'British Birds,' and Bell's 'Keptiles' — such as the 

 venomous and non-venomous snake — and proceed from these low- 

 power magnifications through the whole range up to the delinea- 

 tions of living diatoms as seen with my grand Tolles 1/25 objective, 

 I think you will feel an incipient respect for the use of the little 

 instrument, the use of which I advocate. Just let me call 

 attention to the important fact, that in each rapidly executed copy 

 of an engraving, every mark of the graver's tool has been indicated 

 at one operation by pen and ink while still under the Microscope ; 

 and in mere outlines of microscopic objects — whether executed with 

 pen or pencil, all have been purposely left as they were traced under 

 the instrument ; or to use other words familiar to drawing academies, 

 they have not been " touched up." 



As an example of the satisfactory character of this untouched 

 outline, I hand round copies of the well-known Pleurosigma 



