Aug. 29, 1884.1 



♦ KNOWLEDGE ♦ 



179 



and operating upon extreme heights, commit an error of 

 about one-thirteenth, more or less, of the distance sought. 

 But if, besides the height, we consider (and it is the case 

 here) the breadth of several men, we see that this dimen 

 fion has less variation, and that we could not assign to the 

 error a value of more than one fourteenth. This might, 

 moreover, be sensibly reduced by means of operations re- 

 peated upon different subjects. 



Another cause of error is due to the manner of doing 

 the pacing, which may vary from 0*7 m. to 08 ni. at the 

 most when some little attention is paid to it, or 005 m., 

 more or less, than the normal pace. But the error com- 

 mitted in this case will represent only one thirty-fifth, more 

 or less, of the distance sought, and this may be thrown 

 entirely out of consideration when regulating firing is con- 

 cerned. The two errors, upon being added, will, at the 

 worst, never give a deviation of more than one tenth in 

 the real distance, and we shall admit that such an approxi- 

 mation is sufficiently exact if we reflect upon the gross 

 errors that we should commit in estimating by eye, and 

 ujion the great variations in range that occur in the best- 

 regulated firing. 



Fig. 2. — Apparatus for Estimating Distances (actual size). 



Let us add, that with this instrument it is not necessary 

 to see the entire object ; if the upper part of a man's body 

 can be seen, it will be sufficient. 



In clear weather this process can be applied to distances 

 of 1,000 metres and beyond ; but if there is a field-glass at 

 one's disposal, it will be preferable to use it for very long 

 distances. In all cases it is well to light the image as much 

 as possible in the same manner as the men observed. If, 

 for example, these latter were in shadow and the instru- 

 ment were too brightly lighted, it would be necessary to 



Fig. 3. — Apparatus for Estimating Distances. 



cut off with the hand or cap the solar rays that were falling 

 thereupon. 



It resulted from experiments made at Fort Cagnelot, on 

 the Langre plateau, that, out of thirty measurements, one 

 only could be considered as insufficient, this having given 

 an error of at least one-eighth of the distance. All the rest 

 showed a deviation much less than that which had been 

 fixed on as a limit ; and, if a mean of such deviations be 



sought, it will be found that it was only one twenty-second. 

 The little instrument that we have just described has 

 in nowise the pretension to replace those excellent tele- 

 meters that all infantry corps are provided with ; but it 

 offers a sure and convenient means of estimating distances, 

 and we believe that a frequent use of it will quickly 

 familiarize the observer with making such estimates by the 

 unaided eye, and this, it should not be forgotten, will 

 always be the most really practical method on the field of 

 battle. — La Nature. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Hknet J. Slack, F.G.S., F.R.M.S. 



THE dry season has not been favourable to the growth 

 of fungi, but in cool houses, stoves, and out of doors, 

 some of the gardener's plagues are sure to be found. Here, 

 for example, is a plum-tree against a wall, with leave* 

 looking as if all manner of minute rubbish, plentifully 

 mixed with soot, bad been stuck upon them. An inspec- 

 tion with a hand-glass shows amongst the smut-patches 

 numerous cast skins of aphides ; here and there a small, 

 live object — the so-called red spider, which is a mite — and 

 some scale-bugs. So little of the leaf is left in its natural 

 state that all its functions must be suspended. If an 

 orange-tree is examined, some of its leaves will most likely 

 exhibit similar dark spots. Geraniums in a cool house 

 may here and there have on their leaves the same sort of 

 sooty stuff, and ferns are not infrequently attacked. 

 The only thing to be done for the health of 

 the plants is to wash all the objectionable stufi 

 oflf with soapand-water, or the Gishurst compound — a 

 sulphur soap. If the leaves of any plant exudes honey- 

 dew, or that substance deposited by aphides, black fungi 

 are pretty sure to grow ; but leaves are often covered 

 with them when no particular reason can be detected 

 why they should be assailed. 



If a bit of infected leaf is viewed under the microscope, 

 as an opaque object, small tufts may be discovered, looking 

 less black than they appear to the naked eye. They are 

 composed of thread-like tubes and spores of various 

 shapes, which are translucent enough to transmit some 

 rays of brownish or sometimes olivaceous light. A 

 little of the black stuff may be removed with a pea- 

 knife placed in a drop of water on a glass slide, 

 and spread out by moving a covered glass over it. The 

 annexed figures represent the miscellaneous objects likely 

 to meet the view. It is hopeless for any one not 

 well practised in fungology to name these things with 

 proper discrimination, and great authorities like Berkeley 

 or Cooke would not pronounce upon some except after 

 carefully cultivating them, and seeing what they should do. 

 The old authors called the black stnS Cladosporiiim fumago, 

 but Dr. Cooke tells us it is now usually named Funiago 

 varians, and that it is considered a conidial stage of Capro- 

 dium. If the inquirer examines these things under a 

 microscope, and compares what he sees with the drawings 

 and descriptions of the fungologists, he will think they 

 might be taken for IlehninthosjMrmm, Cladosporium, 

 Macrosporium, and Sporidisinium. 



What is remarkable, and of great though puzzling inte- 

 rest, is the variety of forms under which fungi of the same 

 species can carry on their life processes. In the aggregate, 

 a prodigious amount of work is done by fungi of the 

 minuter kinds. They and their relations — bacteria, &c. — 



