COREESPONDENCE. 187 



or when submitted to the action of dilute acetic acid, exhibit their 

 multinuclear character and granular contents. At certain tui'ns in 

 the tubules the cancer-cells are accumulated in heaps, while in the 

 malpighian corpuscles they are large, subdivided, and contain numer- 

 ous nuclei. It seems probable, then, that the connective tissue cells 

 of the cortical and inter-pja-aniidal portions, and likewise the epithelial 

 cells of the proper renal tissue, are transformed into or replaced by 

 cancer-cells. 



A Neiv Genus of Graptolites. — Mr. J. Hopkinson, F.E.M.S., F.G.S., 

 who gives a very interesting paper on Dicellograpsus in the ' Geolo- 

 gical Magazine' (vol. viii.. No. 1), has used the microscope in his 

 investigations. The author's remarks, which extend to the descrip- 

 tion of five new species, are too long for abstract ; but the paper is 

 full of interesting facts, which show how valuable the microscope is in 

 geology. 



COEKESPONDENCE. 



The Errors of Lenses — Mr. Wenham and Dr. Pigott. 



To the Editor of the ' Monthly Microscojiical Journal.^ 



Ballinamallard Eectort, 



Co. Fermanagh, Feb. 15, 1871. 



SiK, — A well-known philosopher of former times, Scaliger, used to 

 teach that for getting at the truth in a difficulty there was nothing 

 like controversy ; " for as light," he says, " fi-om the collision of flints, 

 so truth is struck out from the collision of minds." Whether this be 

 so or not in general, in the present controversy in your pages it seems 

 in some danger of failing ; for in the last paper of Dr. Pigott the 

 point at issue had been so obscui*ed by things not relevant, that, 

 except to very watchful readers, it may seem doubtful whether any- 

 thing at all has really resulted from the discussion. As, however, the 

 question has in fact been reduced to a very narrow and a very definite 

 issue, it would be a thing to be regretted if this result should be lost 

 sight of. I would, therefore, with your permission, point out how it 

 now stands. 



Immersion lenses being admitted to be superior to the others. Dr. 

 Pigott claims to have discovered the cause of this superiority, the 

 object supposed moimted in a medium like balsam. His theory is 

 that they have a greater aperture. For with the other object-glasses 

 the more oblique rays are lost by total reflexion at the cover ; but 

 with the water-lens a much wider pencil is transmitted to the eye. 

 And in token of this he constructs the elaborate diagram, Plate LX., 

 to show how much exactly is the difference in the angle. By this he 

 claims to have cleared up " the mystery of the water-lens:" 



To the first assertion — viz. that with the common lens the angle 

 cannot exceed 82° — Mr. Wenham replies, this is true; but it is not 



