When the section has been examined the oil must be removed from the lens. For this purpose a soft silk handkerchief or a special piece of chamois leather may be employed, and used very gently. If all the oil cannot be removed, the handkerchief may be moistened with a little absolute alcohol, and the lens hastily wiped. The alcohol must not be allowed to remain in contact with the lens as it is a solvent of Canada balsam with which the lenses are often cemented in position.
Fig. 1.—Double or Triple Nose-piece.
Double or triple nose-piece (fig. 1).—This mechanical arrangement is placed on the lower end of the tube. Two or three objectives of different magnifying power are attached to it. The nose-piece rotates round a central pivot in such a way that the objectives can successively be brought accurately into position above the object on the stage. It is, therefore, a moment’s work to replace a high power objective by a low power one and vice versa. It is an extremely convenient time-saving appliance, and by its use the risk of dropping and injuring the objectives when screwing them on and off frequently is avoided. Those whose microscopes are not already fitted with this appliance can easily have one fitted on at a cost of about a sovereign.
Substage condenser.—This mechanism for concentrating light on the object is a necessity for bacteriological work. The most convenient form is Abbe’s illuminating apparatus (fig. 2).
This consists of a system of short focus lenses which collects the light received by the mirror, and throws it on the object. The amount of light received from the mirror is controlled by an “iris diaphragm,” the aperture of which can be dilated or contracted by moving a small lever at the side. It can be fitted on to most microscope stands, but it is better to get a stand in the first instance which is constructed to carry one.
The cost of a microscope varies from two guineas to two hundred. There are many excellent microscopes in the market, and of these several may be mentioned which the writer has found to work satisfactorily.
Fig. 2.—Abbe’s Illuminating Apparatus.
Of the cheaper student’s microscopes the “Star” microscope made by Messrs. R. and J. Beck, of Cornhill, E.C., will be found a safe investment. It may be obtained with coarse and fine adjustment, nose-piece, and 1 in. and 1/4 in. objectives, for about £5. Those who require a better instrument will find Beck’s “Pathological” microscope fitted with nose-piece, Abbe’s illuminator, &c., for £16, meet all requirements.