332 SUMMARY OP CUREENT RESEARCHES RELATING TO 



couple of fasteners, which may be made of thin sheet brass, to keep the 

 two parts together. There is no necessity for any troublesome 

 grooving and beading to keep out the light ; the pad of blotting- 

 paper put at the back of the plate, to soak up excess of silver nitrate, 

 efficiently does this, and serves also for spring to keep the glass slip 

 in place. A bellows arrangement, to go between the slide and 

 objective, is not so good as a simple piece of black velvet, wrapped 

 round the lens-mount, and extending to the dark slide. This is 

 much less trouble, and more effective. 



We are now prepared for the operation of focusing. It is clearly 

 impossible to do this in the ordinary way. The picture being so 

 small would need a second Microscope to see it, even supposing a 

 surface sufficiently fine could be obtained on which to receive it. 

 The principle of conjugate foci must be made use of, the property of 

 a lens by which the object and its image are always interchangeable. 

 In the dark slide, place face downwards, the thinnest and most 

 distinct microscopic section available, or a micro-photograph. On 

 each side of the centre put a pad of cotton wool or paper, to keep it 

 in place (it is of course a convenience to provide a dark slide with a 

 couple of springs), and close the side. Draw both shutters (which 

 should be marked, so as to show when the hole is uncovered), and 

 place on the stage of the Microscope, section upwards. Let the 

 instrument have a 1 in. objective. Remove the eye-piece, and lay 

 on the top of the tube a piece of ground glass, ground side down- 

 wards. An enlarged image will of course be produced on this ; this 

 must be focused very carefully, and in doing so it is an advantage to 

 use a magnifier. Then, by the well-known property of lenses alluded 

 to above, if an object be placed where the ground glass is, its image 

 will be formed in the place occupied by the section. It is, therefore, 

 quite unnecessary to see the small image. Turn the Microscope so 

 that, on looking along the body-tube from below upwards, white 

 clouds are seen, and replace the ground glass by an ordinary negative. 

 If the operations are carried on on a bench close to a window, the 

 window itself may keep the negative in place, otherwise a retort- 

 stand, or some such thing, must be brought into requisition. Of 

 course it would not entail much trouble to make a special frame, 

 fitting on the end of the Microscope, to carry the negative, but in 

 omitting this the aim has been to enable the worker to see some 

 results as soon as possible. 



Having thus put the instrument and negative into position, take 

 away the dark slide, close both shutters, and insert a sensitive plate in 

 place of the prepared slide, putting at the back a pad of blotting- 

 paper the same size, to absorb superfluous solution, and also to act as 

 a spring. Let the dark slide be now placed on the stage in exactly 

 the same position as before ; and around the objective, and extending 

 from it to the dark slide, to cut off all extraneous light, wrap a piece 

 of velvet, folded once or twice. This is extremely simple, and is 

 done in much less time than it takes to write it. Place a card over 

 the negative, draw the upper shutter, and all is ready for exposure, 

 which is effected by means of the card. Experience must, as usual, 



