COBRESPONDENCE. 189 



object-glass is measiu-ed not by wbat reaches the front but by what 

 reaches the eye. 



Having in this way disposed of his own mistake, he points out one 

 of Mr. Wenham's, which he fancies he has discovered — a mistake which 

 seems to him so odd that it affords him a good deal of amusement, and 

 is in his opinion a legitimate subject for " chaff." In determining the 

 immersion aperture, it seems that Mr. Wenham immerses the front of 

 the glass not in water but in balsam ; thus, as his critic pleasantly 

 says, " demonstrating the principle of the water-lens by applying it 

 tvithout the water to prove its action with it (save the mark !)." Here 

 Dr. Pigott is amused only because he does not understand. In the 

 water-lens there is glass, water, then glass again, the last in contact 

 with balsam. Mr. Wenham, when tracing backwards the course of 

 the ray, to avoid complications not essential, treats the water as if it 

 were a continuation of the glass, jn-operly neglecting the effect of the 

 water on the obliquity because it has no effect, since the ray after 

 leaving the water film recovers its former direction. 



The other parts of Dr. Pigott's paper do not touch the question. 

 The aquariums, the gold fish, Claudius Ptolemy, the bent oar, and the 

 laws of refraction, are nothing to the purpose, though brought in as if 

 they were decisive. It is for this reason I have said that the question 

 was obscxired by the introduction of things irrelevant. The history of 

 Claudius Ptolemy is no more to the point than the history of Claudius 

 Caesar or Claudius Lysias. No more are the laws of refraction and 

 reflexion, repeated (for about the twentieth time,) because these laws 

 are " first truths " and are known to everyone, and never are nor ever 

 have been denied by anyone great or small. There is, no doubt, a 

 sense in which this or any other optical question may be said to depend 

 upon them ; the same sense in which we may say that a question about 

 the nutation of the earth's axis or the motion of the moon's apse 

 depends upon the multiplication table. 



These laws are brought in on every occasion with much arithmetic, 

 and mathematics done with Greek. This display of learning seems 

 to have impressed his readers so much that even skilful observers fear 

 to trust their own faculties against mathematics so original and pro- 

 found. In a former letter I pointed out that those who looked into it 

 might find there was some slight misapprehension about this. No 

 doubt it is very hard for non-mathematical microscopists to think so, — 

 to believe that so much mysterious mathematics can have nothing in 

 it, especially after their nerves have been shaken by meeting such 

 words as Pneumo-spJierical and Hydro-spherical, Aberrameters, Krato- 

 meters, Befractometers, Eklola, and all the rest. But so it is. The 

 whole of this " original " learning from beginning to end, the picture 

 of the bent oar not excepted, is simply copied out of the first pages of 

 the little books for beginners, the primers and horn-books of optics. 

 Examples are given in the primers to be worked out, and Dr. Pigott 

 works out other examples with the numbers changed, — that is all. 

 The " original " part is that he works them with infinitely more 

 trouble than is needed ;* which explains his statement that they have 



* As an example typical of this superfluous work, we may take the case of the 

 mercury globule in pp. 26.5, 266 (vol. iv.). Beginning, as he does, with the primary 



