INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC, 773 



condenser, and (2) the absence of tlie ammonio-sulpliate of copper or 

 alum cells, which are troublesome. 



The condenser consists of an 18-inch Voigtlander photographic 

 objective, about 3 inches in diameter, and is probably the largest ever 

 employed in microphotography. The reason of its selection was 

 simply to avoid heat. It is easy to see that if a 2-inch condenser is 

 regarded as suflScient, the same amount of light could be obtained 

 with a 3-inch, away from the heat focus and thus avoid the effect of 

 focussing the sun's rays on the object and the objective. This 

 practical point has been of great value, and explains the absence of 

 contrivances to prevent the passage of destructive heat. 



Modern Applications of the Microscope to Geology.* — An inter- 

 esting article on this subject is contributed by M, L. Fouque to the 

 ' Eevue des Deux Mondes.' The progress of human knowledge, he 

 says, is not accomplished in a regular and continuous manner, but by 

 starts. Sometimes a man of genius gives a new impulse to science 

 by the power of the divine reflex which animates him, but more often, 

 particularly in experimental researches, each clearly marked impulse 

 of the scientific movement is signalized by the employment of a new 

 method of investigation. Thus the invention of the Microscope was 

 the point of departure of brilliant discoveries in natural history, and 

 each of its improvements corresponded to a period of progress in the 

 development of the science to which it was applied. To-day the 

 manufacture of the instrument has arrived at a remarkable degree of 

 perfection, its magnifying power is enormous, its images are of an 

 extreme clearness, and ingenious arrangements have rendered the 

 instrument more manageable without having lessened precision, and 

 its constructors have known how to adapt it to the special require- 

 ments of each class of research. 



The consequences of these innovations were soon manifest. The 

 study of organized beings took an unexpected turn, anatomy and 

 vegetable physiology were entirely transformed, the domain of the 

 zoological sciences was enlarged beyond conception, and the secrets 

 of life have been explored in their most mysterious functions. 



The application of the Microscope to the examination of the 

 inorganic world took place more tardily in consequence of special 

 obstacles. These difficulties are now happily surmounted. A harvest 

 of new results is being reaped, so rich that it dazzles the imagination 

 of those who gather it. 



In the first part of the article is traced the historical development 

 of modern " Microscopical Petrology," commencing with 1858, when 

 Dr. Sorby's memorable researches first appeared, and on whom is 

 passed a warm eulogium as " the real initiator and propagator of the 

 new method," and after dealing with the labours of Zirkel, Vogelsang, 

 and Eosenbusch the author regrets as a " curious matter and one 

 difficult to explain that though in Germany microscopical petro- 

 graphy is now studied with unequalled ardour, in England, the 

 country of its origin, it seems to make but slow progress." 



* ' Revue des Deux Mondes,' xxxiv. (1879) pp. 40G-31. 



