362 



SCIENTIFIC NEWS. 



[Oct. 5, iE 



to be moved towards or away from the microscope at 

 will. The vertical adjustment is usually supplied with 

 the lamps known as " Bocket lamps." 



The writer, after many trials, has found that the most 

 suitable illuminant is formed by adding to one pint of 

 Defries' oil one ounce of camphor, and allowing it to 

 dissolve. This gives a brilliant and powerful light, very 

 rich in actinic rays, whilst little or no smell arises 

 from it. 



The wick should never be trimmed with scissors, but 

 the charred portion should be rubbed off gently with the 

 finger till it is level with the top of the burner, and all 

 loose threads removed. 



One of the most important details in the whole process 

 of taking a photograph through a microscope must now 

 be considered ; and unless the points now to be men- 

 tioned are carefully kept in view, it is of little use to 

 hope for a sharp negative. 



The slightest error in registration between the focuss- 

 ing screen of the camera and the position occupied by 

 the sensitive plate when in the dark-slide will produce 

 lamentable results. This is the more likely to occur 

 when an inner frame or carrier is used to hold the plate 

 in the slide, and the writer, therefore, strongly advocates 

 that the ordinary ground glass screen of the camera be 

 discarded and that the focussing be done on a piece of 

 ground glass fixed in the carrier in the dark-slide, and 

 secured by means of spring clips, in the exact position 

 which the sensitive plate will subsequently occupy. By 

 this method the chances of faulty registration are reduced 

 to a minimum, whilst by drawing out both the shutters of 

 the dark-slide a capital focussing screen is formed. 



Without dipping deeply into the somewhat complex 

 optical question of objectives, it is perhaps well to men- 

 tion here several matters which influence very essentially 

 the focussing of an object for photographing, having 

 regard to the correction of the ordinary microscope ob- 

 jective. In order that " chromatism " (that is, a coloured 

 fringe round the object under examination) may be 

 avoided, certain corrections are made in the objective 

 supplied for ordinary microscopic research, and as a 

 result the lenses composing it have two foci, known 

 respectively as the chemical — i.e., the point at which the 

 blue rays are brought to a focus — and the visual focus, 

 or the point at which the image of the object under 

 examination appears sharpest, but at which very few of 

 the actinic rays of the light are concentrated. 



It therefore becomes necessary to determine the exact 

 position of the chemical focus, since to the action of 

 these rays is due the record we subsequently obtain 

 on the sensitised plate. A ready method of roughly 

 arriving at this was suggested to the writer. By placing 

 the focussing screen at askew of say ,\ inches on either 

 side of the centre line of the camera, and focussing sharp 

 for the middle of the screen, we shall have in the centre 

 the true visual focus, and on either side an inclined plane, 

 one in front of, and the other behind the visual focus. 

 At some point within these planes the chemical focus of 

 the objective will be found, some part of the plate being 

 more sharply defined than the rest. The position of 

 this zone from the centre should be noted, and the 

 distance in advance of or behind the visual focus deter- 

 mined. Working, however, with such objectives 

 is tedious and uncertain ; and sines at a small cost a 

 correcting lens can be added to the objective, the writer 

 strongly advocates this addition. Only the low powers. 

 3 in., 2 in., ij in., and 1 in., require correction, and the 



additional lens does not impair the value of the objective 

 when required for ordinary microscopic research. In 

 the higher powers, | in., £ in., g in., etc., the visual and 

 chemical foci are practically identical, and any actual 

 difference is best disregarded, since the addition of a 

 correcting lens would greatly reduce the amount of light 

 admitted through the objective. 



Suppose now the lamp to be ready, and that some 

 object suitable for a trial is to be photographed. 



Using the microscope in the ordinary position, the 

 slip on which the object is mounted should be 

 placed on the stage of the microscope and firmly secured 

 by the clips, and an objective with, say, a 1 or 2 in. focal 

 equivalent brought to bear upon it. The object should 

 be brought exactly into the centre of the field, and 

 should be well within its limits. Now place the microscope 

 in its position on the base-board, and carefully bring it 

 to the horizontal position, withdraw the eye-piece, and 

 move the mirror reflector to one side, so that it does not 

 interfere with the light from the lamp, and attach the 

 velvet to the tube. 



A very indistinct image of the object will now be seen 

 upon the focussing screen of the camera, but before 

 sharpening the image the illumination must be so ad- 

 justed that the whole of the field on the ground glass is 

 evenly illuminated. This can, after a little practice, be 

 readily accomplished by moving the lamp nearer to or 

 farther away from the microscope ; but it is well here 

 to remark that uniform illumination can be more readily 

 determined when a medium-sized stop is used in front of 

 the stage than with a larger aperture. The field being 

 satisfactorily illuminated, now sharpen the image on the 

 screen by means of the coarse adjustment of the micro- 

 scope. Should the image bear further amplification, and 

 yet fall within the limits of the plate used, this should be 

 effected by further extending the camera, though the 

 writer strongly advises the use of 4| x 3% plates only, 

 and that the whole of the image falls within a circle 

 2| in. diameter, or a square 2| in. by 2| in., these being 

 convenient sizes for subsequently printing off lantern 

 transparencies from the negatives. 



It is best to remove the eye-piece of the microscope, 

 since it makes but a very unsatisfactory amplifier, and 

 generally introduces distortion of the image, whilst it 

 materially reduces the volume of light impinging on 

 the screen. 



Sufficient amplification can generally be obtained by 

 extending the camera, but should the range be found 

 insufficient, the distance between the micro-objective and \ 

 the focussing screen can be readily increased by first 

 removing the vertical sliding front, behind which in 

 most cameras a sufficiently large aperture exists, and 

 stretching out the velvet cone, or funnel, by which the 

 junction between the microscope and camera is effected, 

 as before described. The desired amplification being 

 obtained, focus the image by means of the coarse and 

 fine adjustments. When focussing for taking a photo- 

 graph care must be taken that the frame in which the 

 focussing screen is held lies in a plane at right angles in 

 all directions to the optical axis. 



(To be continued.) 



Action of Ether upon Vegetable Life. — According 

 to G. Bernstein, an atmosphere saturated with ether kills 

 the sprouts of wheat and barley in about half-an-hour. 



