MANAGEMENT OF THE MICROSCOPE. 



131 



at the base are supported upon castors, so that the platform may run 

 round to each observer in succession. Or the table itself, if not too large, 

 may rotate (like a dumb-waiter) upon its central pillar, as made by 

 Messrs. Beck. 



130. Light. Whatever may be the purposes to which the Microscope 

 is applied, it is a matter of the first importance to secure a pure and ade- 

 quate Illumination. For the examination of the greater proportion of 

 objects, good daylight is to be preferred to any other kind of light ; but 

 good lamplight is preferable to bad daylight, especially for the illumina- 

 tion of opaque objects. When daylight is employed, the Microscope 

 should be placed near a window, whose aspect should be (as nearly as may 

 be convenient) opposite to the side on which the sun is shining; for the light 

 of the sun reflected from a bright cloud is that which the experienced 

 Microscopist will almost always prefer, the rays proceeding from a cloud- 

 less blue sky being by no means so well-fitted for his purpose, and the 

 dull lurid reflection of a dark cloud being the worst of all. The direct 

 light of the sun is far too powerful to be ordinarily used with advantage, 

 unless its intensity be moderated, either by reflection from a plaster of 

 Paris mirror, or by passage through some 'Modifier' (109) ; it is, how- 

 ever, occasionally used by some observers to work out intricate markings 

 or fine color, and may sometimes be of advantage for these purposes, but 

 without great care would be a fertile source of error. The young Micro- 

 scopist is earnestly recommended to make as much use of daylight as 

 possible ; not only because, in a large number of cases, the view of the 

 object which it affords is more satisfactory than that which can be obtained 

 by any kind of lamplight, but also because it is much less trying to the 

 eyes. So great, indeed, is the difference between the two in this respect, 

 that there are many who find themselves unable to carry on their obser- 

 vations for any length of time by lamplight, al- 

 though they experience neither fatigue nor strain 



from many hours' continuous work by daylight. 

 Even ordinary daylight may be considerably im- 

 proved by the interposition of a . glass globe of 

 about six inches in diameter, filled with water; and 

 this may also be advantageously used for the illu- 

 mination of transparent objects by lamp-light, if 

 the water be very slightly tinged with ammonio- 

 sulphate of copper, which takes off the yellow 

 glare . 



131. Lamps. When recourse is had to Artifi- 

 cial light, it is essential, not only that it should be 

 of good quality, but that the arrangement for fur- 

 nishing it should be suitable to the special wants 

 of the Microscopist. The most useful light for 

 ordinary use is that furnished by the steady and 

 constant flame of a flat- wicked Lamp, fed with one 

 of the best varieties of Paraffin oil. This (with its 

 chimney-shade) should be so mounted on a stem 

 rising from a secure base, as to be capable of ad- 

 justment to any height above the table; and on the same stem should also 

 slide a telescope-arm having a bull's eye condenser attached to it by a 

 ball-and-socket joint, in such a manner as to be adjustable in any posi- 

 tion in regard to the flame, and at the same time to be carried upwards 

 or downwards with the lamp an arrangement originally devised by Mr. 



