56 



HARDWICKE'S SCIENCE-GOSSIP. 



same distance above the stage, and hence never 

 moves out oi focus. And, because, in every sphere, 

 a horizontal circle has always a vertical axis, the 

 centre of the sphere must be vertically above the 

 centre of the circular aperture of the stage ; hence, 

 no amount of rotation of the hemisphere can move 

 the object out o'i field. The specimen, then, can be 

 turned about in all sorts of ways, without the faintest 

 chance of the observer being inconvenienced by its 

 shifting, either out of field or focus, and, at the same 

 time, permitting the use of objectives of fairly hij;h 

 powers. 



By turning out a conical hole of the size and 

 shape shown in Fig. 28, this simple contrivance may 

 be employed in transmitted light, for adjusting 

 sections of crystals, which are supposed to have 

 been cut in any particular direction; but, which, as 

 is almost always the case, only approximate what 

 they are represented to be. Suppose, for example, a 

 section of a doubly refracting, uniaxial crystal is 

 required, normal to the optic axis. Then, by em- 

 ploying the hemisphere, the section can be so 

 adjusted that its optic axis is exactly parallel to the 

 line of collimation of the instrument. 



Another simple addition I have found useful in 

 the examination of imperfectly transparent mineral 

 fragments. It consists of a hollow cone of blackened 

 cardboard, made somewhat after the style of a Bunsen 

 cone. The cone may be so placed that its wider end 

 rests upon, and is concentric with, the stage of the 

 microscope ; whilst its narrower end above, loosely 

 encircles the objective and allows of a limited 

 amount of focussing down. Thus the stage with 

 the cone may be freely and uninterruptedly revolved, 

 and, at the same time, every trace of reflected light 

 cut off. This appliance, which can be made of 

 varying sizes to suit the various objectives employed, 

 is especially useful in examining well-worn sand- 

 grains of, for example, some pleochroic mineral, 

 when it is obviously essential to be rid of all re- 

 flected light, seeing reflected light, would, in any 

 case, be prejudicial to a correct observation of pleo- 

 chroism, and in some cases, where the specimen is 

 but imperfectly translucent, be sufficient to com- 

 pletely eclipse tlie very feeble amount of transmitted 

 light. 



N'orinal School nf Science, SoiilJi Kensington. 



ABNORMAL GROWTHS ON FOREST- 

 TREES, 



The Beech Knot. 



AMONG the many curious and abnormal growths 

 which are found on branch, root, and stem of 

 our forest trees, the above, the beech knot, is 

 among the many curious and interesting. 



1 have of late, and on previous occasions, observed 

 in some journals and papers a short notice of the 



above growths, attributing their origin to fungus^ 

 insects, accident, and such like. I have recently 

 collected many specimens of the above abnormal 

 growths and other tree excrescences and diseases. I 

 have made a close study of them in many woods on 

 many large trees, on various extensive properties 

 where I have been engaged. 



They are not previously treated nor illustrated in any 

 journal, as far as I am aware ; after a careful study of 

 them, I shall, with thehelp of the following illustrations, 

 try and interpret, in an intelligible way, what I consider 

 to be the cause of their origin and after-development. 



I have termed them the Beech Knot, because 

 they are most numerously found upon that tree ; 

 but we find them common upon the oak, ash, elm^ 

 alder, chestnut, holly, evergreen oak. They are not 

 due to insects, fungus, nor accident ; but are perfectly 



natural. Neither may they be taken as an indicatiorr 

 of health nor disease, nor are they in any way 

 attributable to any particular soil or situation. They 

 are perfectly numerous on some trees ; on others 

 comparatively rare ; some die off early and rarely 

 attain the size of a pigeon's egg, while I have 

 found some really large and remarkable specimens. 

 How are they caused, is the main inquiry, and 

 particularly, how do they grow ? In the barks of our 

 forest trees are contained a multitude of latent buds 

 which are developed and grow under certain favour- 

 able conditions. Some trees possess this property in 

 a remarkable degree, and often when the other parts 

 are killed down by frost in severe winters, this 

 property of pushing out these latent buds into growth, 

 often preserves the life of the plant. 



