September, 1903.] 



KNOWLEDGE. 



213 



ordinary pocket lens at any moment to ascertain if anything 

 has been secured worth preserving. 



It is advisable to carry as many bottles as the number of 

 ponds that are likely to be visited ; careful record should be 

 kept of the exact locality where each mite is found, with the 

 date of capture, and this cannot be done if all the specimens are 

 carried home in one bottle. 



The most convenient way of carrying collecting bottles is by 

 sewing two strips of thick cloth together with loops of the 

 required size in the same manner as a cartridge bandolier. Such 



Fio. 3. 



a device can be rolled and stood at the bottom of a bag, and 

 obviates the cbame of the bottles breaking by contact. 



During the summer months it will generally be found that 

 the most successful captures are made near the edges and in 

 shallow parts of ponds ; in the winter time the mites get into 

 deeper water. Some mites are to be found only on the mud at 

 the bottom of ponds, others on the leaves and stems of water- 

 plants. In collecting, therefore, it is necessary to let the edge 

 of the net just skim over the surfice of the mud and sand, and 

 u]) and down the stalks and stems of likely plants. 



The under-Burfaces of leaves should also be scraped with the 

 edge of the net. Anacharis is a very favourite plant of water- 

 mites, and wherever this is found it is almost certain that mites 

 will be secured. 



In addition to the free-swimming mites, there are a large 

 number of parasitic forms, and it is as well to examine all forms 

 of insect-life before discarding material. Fresh- water mussels, 

 in particular, also the large water-snails and water-beetles, are 

 specially to be recommended for examination, and, once more, 

 let me emphasize that if anything is found, notes should be 

 made of dates, places, and general details of the captures. 

 (To be continued.) 



Focussing in Piioto-Micrograpiiy. — There are seveial 

 devices in regular us^e which enable the object that is to be 

 photographed to be viewed and focussed without reference to 

 the ground-glass screen; among these are "Becks Observing 

 Prism," and Mr. Andrew Pringle's device for talcing instan- 

 taneous jihotographs, which is described in his "Practical Photo- 

 Micrography." But it would often be an immense convenience 

 to the worker if, after focussing the object through the 

 microscope,',he moroly had to set his c.inera in position and 

 make the exposure, without readjustment in the camera itself. 

 For many classes of work this is almost a necessity, and a 

 suggestion offered by a contributor to the Journal of Applied 

 Microscopy is an excellent and practical one. 



Although the difference is small between the point of visual 

 focus when examined directly through the eyepiece and when 

 seen upon the ground-glass screen, it can be exactly adjusted 

 by the use of a spectacle lena. This spectacle lens has to suit 

 the individual user, the particular objective, and camera length, 

 but with a variety of these glasses any condition may be 

 provided for. 



The simplest way would bo to fix standards by first of all 



focussing the object sharply upon the ground-glass screen, re- 

 moving the camera, and then, looking through the microscope eye- 

 piece in the ordinary w^ay, interpose between it and the ej'e (but 

 particularly without altering any of the focussing adjustments) 

 several of these spectacle lenses. When the one has been found 

 that shows the object sharply and at its best, it may be retained 

 for the special purpose in future, i.e., the next time the same 

 objective and camera length is to be used, after the usual 

 focussing adjustments are made with the microscope, this 

 ])articular lens will be interposed and the focussing altered the 

 slight amount necessary. The camera will then be placed in 

 position, and there will be no occasion to refer to the ground- 

 gla.ss screen to verify the focussing. 



This system is good so far as objects are concerned with 

 which eyepieces are used, but aid is often more particularly 

 required where " Planar " and long focus lenses are employed 

 without eye])ieces. It is usual to insert an ej-epiece to place the 

 best part of the object in the centre of the field, then remove 

 the eye])iece and re-adjust upon the ground-glass screen ; but 

 the difference between the two points of focus is very great, 

 and with considerable camera length difficulty is often 

 experienced in reaching the microscope adjustments : a hint in 

 this direction may therefore be of value. 



When observing with the eyepiece, a plano-convex lens of a 

 focus which will vary according to the lube length and other 

 considerations should be inserted in the lower end of the draw- 

 tube. The object is then focussed with this in position. For 

 photographing, the draw-tube is entirely removed together with 

 the viewing lens and ej-epiece, and if the same method of 

 procedure is followed in this instance as in the one previously 

 described above, there will be no occasion to re-focus on the 

 ground-glass screen; all that will be necessary will be to view 

 the image sharply focussed on the ground-gla«s screen experi- 

 mentally in the first place, then find the lens which, when placed 

 at the lower end of the draw-tube and the object viewed 

 through the eyepiece, will give ihe same image sharply focussei 

 to the eye without altering the adjustment-. 



Orjectives for Poxii-LiFE Ex.\.Mi.NATioK. — A Sup])lemen- 

 tary Note by Mr. C. F. RorssELET. — A few words on the 

 most suitable object glasses for pond-life work may be 

 useful to the uninitiated. It goes without saying that only the 

 very best objectives will give permanent satisfaction. As low 

 powers, the li in. and ijin are the most useful glasses and quite 

 sufficient for all work, the jj in. being, in my opinion, the highest 

 power which can usefully, and with real advantage, be used 

 with the binocular microscope. Then, for the study of the 

 minuter structure of the animals, a higher power, giving a 

 magnification of at least 3l)) diameters, is required. A 

 thoroughly good \ in. or jV in. object glass will answer this 

 purpose. Both the.se glasses, however, have a very short 

 working distance, which is a source of considerable trouble 

 when working on living and moving objects in the compressor. 

 In practice I have found Zeiss' VI mm. (or '. in.) apochromatic 

 objective far better suited for this i)Urpose. It has a good 

 working distance of about jjth of an inch, and yields a most 

 perfect image with the various compensating eyepieces, ranging 

 from 84 to :S78 diameters. This will cover most of the work 

 the amateur is likely to do ; but occasion.ally, for exceedingly 

 minute anatomical details, an object glass of greater numerical 

 aperture will be required, for which I recommend a ^ in. or^L in. 

 aiiochromatic water-immersion lens. 



NOTES AND QUERIES. 



(1. Damant. — I quite understand your difliculty, but it is 

 practically impossible to obtain the light from one "spot'' if an 

 incandescent filament is used. I ain afraid you will have to 

 Siicrifice the ideal in this res])eot. An inamdescent bulb could 

 bo quite simply mounted ou a vertical standard to do what you 

 require, and very good results could be secured. 



NOTES ON COMETS AND METEORS. 



By W. F. De.\.\ing, F.R..\.s. 



Borbki.ly's Comkt formed a fairly conspicuous object in July, 

 passing successively tlu-ougli Cygnus, Draco, and Ursa Major, 

 but it has now iipproaehed too near to the sun for observation. Even 



