lo Nov., 1910.] Tlic Spread of Weeds and of Plant Diseases. 



damaged and are affected by atmospheric conditions and sometimes broken, 

 and replacing them is a tedious job even to an expert. The "eye" 

 end of the telescope is fitted with a small microscope which is slid in 

 and out of its tube by the hand with gentle force and with rotary motion. 



Point the telescope to some well 

 defined object, such as a house or tree, 

 or to the sky, and operate this micro- 

 scope until the cross-hairs appear quite 

 clear and devoid of any apparent shake. 

 At the same time, focus the telescope 

 Telescope fube also by means of the milled head screw 

 at its side so that clear vision of the 

 object observed is obtained. It would 

 be well to do this before levelling the 

 instrument to avoid disturbing it, and 

 as a rule, once the cross-hairs are in 

 focus the microscope will not require 

 any further attention during the day's 

 work. A very considerable range of tem- 

 perature, or sudden change from sunlight to cloudy conditions, will be 

 the most frequent causes of re-focussing. The telescope will require 

 focussing for nearly every reading of the staff. 



All readings are made with the horizontal hair; the staff should be 

 read between the vertical hairs. If there is any suspicion that these 

 hairs are not vertical, sight to a fine string carrying a heavy phimbob 

 and suspended where it is not affected bv wind. If in error, correct 

 by loosening the diaphragm screws, when the diaphragm may be moved a 

 little to right or left as required and with a rotary motion, the holes 

 in the telescope tube through which the screws pass being slotted for the 

 purpose. Doing this will derange the adjustment of the instrument so 

 it will be wise to verify it prior to adjusting. When these hairs are truly 

 vertical, the horizontal wire will be true also. 



{To he continued}) 



D/ofihro^m 



16. DIAPHRAGM OF TELESCOPE. 



THE SPREAD OF WEEDS AND OF PLANT DISEASES. 



Alfred J. Eivart, D.Sc., Ph.D., F.L.S., Government Botanist and Professor 

 of Botany in the Melbourne University. 



There are many facts in regard to the spread of weeds and of plant 

 diseases which are little known, and others, the precise bearing of which 

 io not generally correctly understood. Hence, it may be of interest to 

 give a general account of the chief factors responsible for the spread 

 of vegetable pests and at the same time to mention a few illustrative 

 cases which are of special interest or which have come lately into 

 prominence. 



The old idea in regard to pests and plagues, whether of animal or 

 vegetable origin, was that they were to be regarded as visitations of God 

 to punish the wickedness of man. This view is well illustrated in the 

 Biblical accounts of the plagues of Egypt. Such puni.shment, of course, 

 falls on both innocent and guilty alike, and .sometimes indeed with greater 

 weight upon the former. Among Mahomedan countries, the old idea is 



