132 CORRESPONDENCE. 



Minister of Public Instruction for the chair of Comparative Embryo- 

 genesis at the College of France, were balloted for. Tbe names of 

 MM. Gerbe, Balbiani, and Dareste, were presented to the meeting, 

 and the result of the voting was to select the two former gentlemen 

 as the Academy's nominees for the post. 



An Inorganic Mimicry of Organic Life.— Mr. J. Sidebotham, 

 F.R.A.S., exhibited at a late meeting of the Microscopical Section of 

 the Manchester Philosophical Society (January 19th) some curious 

 specimens illustrative of this process. The title of the paper which 

 he read upon the subject was " The similarity of certain Crystallized 

 Substances to Vegetable Forms." In this he called attention to the 

 formation of verdigris on insect j)ins, in old entomological collec- 

 tions. This substance makes its api^earance where the pins pass 

 through the thorax of the insects, and in length of time grows into a 

 considerable mass of flocculent matter, of a brilliant green colour, and 

 often breaks up the insects and also destroys the pins. It consists 

 mainly of acetate or formiate of copper in combination with fatty or 

 oily matter. On examination of various specimens tmder the micro- 

 scope, they were found to present a great variety of forms, filamentous 

 and ribbon-like structure, often resembling various fungi, in some 

 cases so nearly, that it was difficult to believe that the fibres and 

 fruit-like forms are not really organic bodies. Mr. Sidebotham 

 expressed his opinion that these bodies were simply crystals, modified 

 in their formation by the oil contained in the insects, with which the 

 crystals are in some way combined. Some of the specimens exhibited 

 were taken from insects collected twenty-five years ago. 



COKEESPONDENCE. 



Mr. Keith's Examination of Mr. Tolles' Objective. 



To the Editor of the ' Monthly Microscopical Journal.' 



Georgetown, D.C, January 20, 1874. 



Sir, — My brief note to Dr. Woodward in connection vidth his trial 

 of Mr. Tolles' objective, published in your Journal, does not seem to 

 have conveyed fully my purpose in alluding to the additional lens. I 

 wish to explain a little more fully my view of the matter. 



All see that the limit 82^ depends upon the difference of refractive 

 power at a plane surface ; that is, upon two variables ; and, therefore, 

 necessarily changes with a change of either of them. 



If balsam is substituted for air between the objective and the cover, 

 the refracting surface is practically removed from the cover to the 

 posterior surface of the front lens, from a plane to a curve, and the 

 limit, which depends upon the curvature, changes with that variable. 



If water is substituted for air between the objective and the cover. 



