HA RD WICKE'S SCIENCE- G OS SIP. 



265 



THE RE-AFFORESTING OF LONDON. 



By E. A. PEAK. 



^HILE cordially 

 agreeing with Mr. 

 Mattieu Wil- 

 liams's remarks 

 on the desirability 

 of this object, and 

 the suitability of 

 the plane for the 

 purpose, I am 

 unable to under- 

 stand the advan- 

 tages claimed for 

 it, by reason of 

 the shedding of 

 its bark ; neither 

 do I think that 

 the suggested test 

 of its vitality, so 

 far north as Shef- 

 field, would be 

 at all fair to the 

 plane, and I question if any practical nursery- 

 man would so recommend it. Bentley says : " The 

 new layers of. wood, the cambium layer, and the 

 recently-formed liber, are the parts of an exogenous 

 stem which are alone concerned in its active develop- 

 ment and life." And I am inclined to think that its 

 success as a town tree may be due more to its large 

 smooth leaves, which prevents an accumulation of 

 dirt from finding a lodgment, than to the shedding 

 of the bark. 



In 1860-61, twenty plane-trees were planted in the 

 park here, which speedily grew into fine, healthy, 

 smart-looking trees. During the winter of 1S70-71, 

 which was preceded by a wet autumn, the thermo- 

 meter got down as low as 5 or 27 of fro=t, and 

 most of them were killed, the remainder being much 

 injured ; the latter, however, made ■ what may be 

 called desperate efforts to recover themselves, and 

 continued to do so, with more or less success, until 

 the winter of 1880-81, when, under similar climatic 

 conditions, every one was killed. 

 What may we learn from this ? 

 The plane always grows until late in the autumn. 

 No. 264.— December 1886. 



Its wood has rarely time to properly ripen, which, in 

 warm, wet, autumns, is intensified, and if a severe 

 winter follows, injury or death is inevitable. 



I may add we have only one healthy well-formed 

 plane-tree of any size in the neighbourhood, and a 

 few young ones in the streets, that are, as yet, doing 

 fairly well ; the remainder are in a wretched condition. 

 It must be remembered that these remarks apply to 

 the north of England only. 



Hull. 



THE TWO MIRRORS. 

 By W. J. N. 

 No. III. 



HAVING acquired some knowledge of the 

 behaviour of parallel rays after reflection from 

 a concave mirror, we have next to consider how that 

 knowledge may be made practically useful to us as 

 microscopists. 



On the table let us place the microscope ; beside 

 it the adjusting lamp, with blue glass chimney, and 

 the bull's-eye ccndenser. On the stage of the instru- 

 ment let us place some transparent object, and then 

 set ourselves to illuminate the object by means of a 

 pencil of rays to be first rendered parallel, and after- 

 wards brought to a focus on the object by the concave 

 mirror ; it being assumed that the illuminating rays 

 are to reach the object with no greater obliquity of 

 direction than such as necessarily arises from their 

 convergence (after reflection) to a distant focus. 



Theoretically, we know that our purpose will be 

 accomplished if we set the mirror in the axis of the 

 microscope at the correct distance from the stage, 

 and then throw upon it a pencil of parallel rays at a 

 small angle of incidence, such as 30° (see last Art.). 

 But in order to carry the theory into practice, it is 

 evidently necessary to determine three things : 



1. The correct distance in question, between object 



and mirror, to give a true focus. 



2. The correct position and height of the lamp, to 



give the desired angle of incidence to the 



illuminating pencil. 



N 



