PROCEEDINGS OF SOCIETIES. 203 



with two lenses of the same refracting medium, by combining a bi- 

 convex and a meniscus lens, as shown by Herschel. 



Lastly, a reflecting telescope has its principal reflector worked to 

 a parabolic curve, . to avoid the spherical aberration that would be 

 introduced were the reflecting surface spherical ; but though compoimd 

 light is used, there is no chromatic aberration to correct, whether the 

 reflecting surface is sjiherical or parabolic in its curve. How then 

 these two errors can be regarded as " identical in character," is to me 

 simply incomprehensible. 



Dr. Eoyston-Pigott complains, and with reason, of the scant 

 courtesy of tone in some of the correspondence which has appeared in 

 this Journal upon optical questions ; but surely he should not include 

 in his condemnation the letter of our old friend Mr. Chas. Brooke 

 which appeared in No. Ixxxv., p. 45, in which I can perceive nothing 

 but a temperate and legitimate protest against what the writer regards 

 as erroneous doctrine uj)on a scientific matter. 



I shall be very sorry if Dr. Eoyston-Pigott regards my letter in 

 the same light, as nothing can be farther from my thoughts than 

 offering him any offence ; but unless adverse criticism of novel 

 theories can be freely indulged, scientific progress must be impeded. 



I am, Sir, your obedient servant, 



Geo. Shadbolt. 



PEOCEEDINGS OF SOCIETIES. 



EoYAL Microscopical Society. 



King's College, March 1, 1876. 



H. C. Sorby, Esq., President, in the chair. 



The minutes of the preceding meeting were read and confirmed. 



A list of donations to the Society was read by the Secretary, and 

 the thanks of the meeting were voted to the donors. 



The Secretary said they had received a paper " On the Measurements 

 of the Diatoms of Moller's Probe-Platten," by Professor Morley, which 

 would be printed in the Journal. The paper was accompanied by 

 a number of tables, copies of which had also been sent for distribution 

 amongst the Fellows present. 



Mr. W. N. Hartley, F.C.S., read a paper " On the Identification of 

 Liquid Carbonic Acid in Mineral Cavities," illustrating the subject 

 by drawings upon the slate, and by sj)ecimens exhibited under the 

 microscopes in the room. Two specimens were so arranged that a jet 

 of hot air could be impinged upon them whilst under observation, and 

 the sudden vaporization and subsequent condensation of the liquid 

 enclosed in the cavities was in this manner clearly demonstrated. An 

 ingenious contrivance was also exhibited by means of which the 

 critical point of a liquid could be readily determined. 



The President felt sure that the Follows would give a very hearty 

 vote of thanks to Mr. Hartley for the very interesting paper which he 

 had just read to them. He was very much gratified to find that 



