CORRESPONDENCE. 153 



When Mr. Tolles' glass was handed to me, I at once noticed the 

 neatness and accuracy of the workmanship ; but it must be borne in 

 mind, that in this particular case the glass was heralded long before 

 its arrival with the announcement that it was to prove a peculiar 

 condition or advantage which no English glass would be found to 

 have. Naturally a number of microscopists were curious to see its 

 performance, and comparisons became inevitable. The variety of 

 test objects used were such as are usually sold, mounted under covers 

 something near -^^-^ thick. I admit that I did not try anything as 

 thick as yg^h ; few glasses of this j)ower are made for such covers, 

 I am therefore quite ready to allow that, under these conditions, the 

 performance might have been different. 



Your obedient servant, 



¥. H. Wenham. 



Inexperienced Artists v. Experienced Ones. 



To the Editor of the ' Monthly Microsmpical Journal.^ 



London, Aug. 11, 1873. 



SiE, — Amid the numerous singular letters which have appeared in 

 " re Pigott," none are more singular than that by Mr. B. D. Jackson, 

 in your last number. 



Mr. Jackson evidently mistakes the reasoning of the two authors, 

 one of whom is estimating the value of positive and the other of 

 negative evidence. 



Dr. Pigott's argument may thus be put. 



If a person, totally unacquainted with an object or a subject in 

 dispute, make a drawing clearly depicting certain structiire stated by 

 A. B. to exist, it ought, a fortiori, to be clear as noonday to the prac- 

 tised observer, and it necessarily follows, if the unskilled observer 

 represent with his pencil that which A. B. has described with his pen, 

 that the latter has not drawn on his imagination. 



The argument in Schleiden is exactly the reverse of this. 



If a person, ignorant of the structui'e of an object, make a drawing, 

 omitting certain well-known organs or developments, it is not to be 

 inferred that such structure does not exist. On the contrary, such 

 omissions are in the highest degree probable, and therefore such draw- 

 ings are, from their probahle omissions, valueless. 



As a matter of positive evidence none can be stronger than that 

 of a disinterested witness, and our late President, Mr. Keade, tells us 

 in his jjaper " On the Diatom Prism " that he had the " evidence of 

 an unprejudiced witness," for a boy, looking at Formosum through 

 his instrument, saw what looked like a " plate of marbles" — this was 

 conclusive as to the optical appearance. 



Let us suppose that in Central Africa Dr. Livingstone is assured 

 by a savage tribe that a civilized stranger has passed that way, the 

 evidence will be unsatisfactory by itself; but let these " Inexpe- 



