cements, canada balsam

Значение термина cements, canada balsam в knolik

1. This material forms a very useful cement for many purposes. It is the only cement employed by opticians for uniting the lenses of achromatic objectives. For this purpose, it must be pure and colourless. It is easily bleached by exposure to sunlight. If too thick, it may be thinned with benzole. In cementing the two parts of an achromatic lens together, the surfaces should be thoroughly cleaned, and the glasses, having been previously warmed, should be laid on some surface which will not scratch them. By means of a rod of glass or metal, place a drop of balsam on the centre of one lens, and then gently lower the other down upon it. Now apply a slight pressure, and the dark disc in the centre, indicative of optical contact, will rapidly increase in size, until at last the balsam reaches the margin and begins to ooze out at the edges, if the balsam be in excess, as it ought to be. By means of a piece of soft string, if the lenses arc large, or a spring clip, if they be small, the lenses should be held firmly together and exposed to a gentle heat in an oven that is cooling, or before a fire until the balsam at the edges has become hard and dry. The string or clip may then be loosened, and all external traces of balsam removed, first by scraping, and afterwards with a little benzole or ether. The above directions, modified to suit circumstances, apply to the cementing of glasses for transparencies or opal pictures; also to the varnishing of magic-lantern slides, and the protection of any transparent surfaces from the air.
2. Canada balsam forms a very efficient and easily applied cement for the construction of small tanks used by microscopists for keeping minute plants and animals alive in water.

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