6 PRESIDENTIAL ADDRESS. 



If the two Nicol's prisms — the polariser and the analyser — be 

 placed parallel to each other, the components of the light, which 

 are transmitted by the polariser, are polarised in the right direction 

 to be transmitted by the analyser, and we get a bright field of 

 view. But if the polariser is turned through a right angle, the 

 plane of polarisation of the light transmitted by it will be that of 

 the more oblique beam, which is totally reflected by the balsam in 

 the analyser, so that the whole of the light will now be extin- 

 guished and the background will be dark. In this case the Nicol'g 

 prisms are said to be crossed. 



If any object be now placed on the stage of the microscope, 

 and if it be of such ^ nature that the light continues polarised in 

 the same plane after passing through it, this light is still arrested 

 by the analyser and the object remains invisible. If, therefore, we 

 define 2. polar iscopic object 2S one which is visible when the polari- 

 scope gives a dark ground, we see that polariscopic objects must 

 possess the property of affecting a beam of plane polarised light 

 in such a way that after passing through them it is no longer 

 polarised in the same plane. 



Vascular Tissues. — I should be glad of a short Hst of widely 

 distributed plants, which are most suitable for showing spiral and 

 annular vessels, pitted ducts, etc. In one of the editions of 

 Carpenter's " Revelations," there is a woodcut of a longitudinal 

 section of what he calls " Italian Reed," which shows very clearly 

 and beautifully, in one bundle, all the above forms of vessels and 

 ducts. I should be glad to know more about the reed in question. 

 If not a native plant, as the name would suggest, how may it be 

 procured ? An answer by a botanical reader would much oblige. 



R. W. A. 



King's Fluid. — What is known in America as King's fluid for 

 algae is really Petit's, but as the Rev. Mr. King introduced it, it 

 was called by his name. The formula is as follows : — Take 

 Camphor Water, 50 grms. ; Distilled Water^ 50 grms. ; Glacial 

 Acetic Acid, 0*50 grms. ; Chloride of Copper Crystals, 0*20 grms.; 

 Nitrate of Copper, 0-20 grms. V. A. L. 



