796 sujmmaky of current researches relating to 



into ammonia ; and evt-n the same organisms, according to the conditions, 

 may either have an oxidising or a reducing function. In the first phase, 

 when the nutritive matter is readily oxidizable and assimilated, the micro- 

 organisms thrive at its expense, the process of nitrification being materially 

 assisted by atmospheric oxygen ; in the second phase, on the other hand, 

 the necessary oxygen is derived from the nitrates ; thus a change, seemingly 

 of reduction, is induced. 



MICEOSCOPT. 

 a. Instruments, Accessories. &c.* 



(1) Stands. 

 Thury's Multiocular Microscope. — Prof. M. Thury has devised a 

 Microscope (figs. 201 and 202) for enabling several observers to view the 

 same object without having to change their seats. The following is a 

 translation of the description which he sends us : — 



"It is well known how tedious demonstrations with the ordinary Micro- 

 scope are in consequence of the professor and his pupils having to con- 

 tinually change places. The Microscopes vy^ith two or three tubes, designed 

 by M. Nachet (figs. 203 and 204) [and that of Prof. Harting,! fig. 205] 

 obviate this inconvenience, but at the expense of a deterioration of the 

 image, which is the more objectionable in consequence of its increasing 

 rapidly with the power of the objective. This essential defect arises from 

 the injurious influence of the edge of the prism, always imperfect, which 

 occupies a diametral position relatively to the objective, disturbing a zone 

 in the latter of constant size, which Las therefore a greater influence on the 

 image according as the objective has a smaller diameter. 



It seemed to me that these inconveniences might be avoided, if in place 

 of dividing the image between different observers it was received entire by 

 a total-reflection prism, and by a movement of the prism passed succes- 

 sively to the difl(.'rent oculars of a Microscope with several tubes. 



In consequence of the aberrations of colour and form which always take 

 place in the case of prisms, the reflected image cannot be as perfect as the 

 image obtained without it, but the difference is hardly appreciable. For 

 instance, a Hartnack No. 9 objective which shows the beads of Pleurosigma 

 angulatum with central light in the ordinary Microscope, shows them also, 

 a little less distinct only, after reflection by the prism. A mirror of silvered 

 glass would remedy this defect but at the expense of diminished permanence 

 of the reflector. 



The position of the prism P is shovm in fig. 202. It is placed at a 

 little distance behind the objective so as to diminish the effects of aberra- 

 tion, which are at their maximum when the prism is immediately behind 

 the objective, as is necessarily the case when the image is multiplied in 

 the manner hitherto adopted. The stage of the Microscope being horizontal 

 and the optic axis of the objective consequently vertical, the prism is 

 arranged to turn round a vertical axis situated in the prolongation of the 



* This subdivision contains (1) Stands; (2) Eye-piecee and Objectives; (3) Illumi- 

 nating and other Apparatus ; (4) Photo-micrography ; (5) Microscopical Ojitics and 

 Maniiiulation ; (G) Miscellaneous. 



t Harting, P., Das Mikroskop, 1850, p. 780 (1 fig.). 



