xxii INTRODUCTION. 



better ' test objects' than the markings of diatoms. It is most important to have a good high 

 power such as Hartnack's No 7, equal to about an English \ inch. If a higher power be 

 required, Zeiss' E = ^ inch, and his F = -j l j- inch, are excellent lenses. 



IMMERSION LENSES. 



Ordinary lenses are ' dry,' that is, air is the medium between the object and the objective. 

 For higher powers, lenses are specially constructed so that a fluid medium is placed between the 

 object and the objective. The effect of this is to collect a larger number of rays of light 

 passing through the object, and so increase the illumination of the field of the microscope. 

 Water was formerly employed, and lenses were specially constructed for use with water 

 as the medium. Recently, however, oil immersion lenses have been constructed by Zeiss of 

 Jena, and his -^ inch is the best lens. These lenses are rapidly taking the place of water 

 immersion lenses. Similar lenses are made by Messrs Powell and Lealand. Cedar oil is the 

 medium employed for these lenses. A drop of oil is placed on the lens, and the tube is then 

 lowered till the oil comes in contact with the slide, and the object is focussed in the ordinary 

 way. They increase the sharpness and brilliancy of objects immensely. As the cedar oil 

 dissolves dammar, all preparations to be examined with oil lenses must be sealed up with 

 marine glue, which is not acted on by cedar oil. It is not necessary for the student to get 

 these expensive lenses ; they are supplied when they are required. 



A nosepiece is a simple device (fig. 6) by means of which the unscrewing of one objective 

 to use another is obviated. Either the high or the low power can be used. It is necessary 



that both lenses be accurately centred. The nosepiece is 

 screwed to the tube of the microscope, and to it the lenses are 

 screwed. A nosepiece may likewise be made to carry three 

 or four lenses. 



Eyepieces. Nos. 3 and 4 are supplied with Hartnack's 

 microscope but it is well always to use the deep eyepiece, 

 or No. 3, i.e. the less powerful of the two. 



THE ADJUSTMENTS OF A MICROSCOPE. 



In every microscope there are two adjustments : 

 (a) The coarse adjustment, which serves to bring the 

 Fig. 6. NOSEPIECE MADE BY SWIFT. ob j ect roughly within focal distance. In Hartnack's micro- 

 scope, this is accomplished by rotating not pushing one 



tube inside another with the hand. This is by far the most convenient method. As the 

 tube of this microscope is not cloth-lined, the two surfaces ought to be kept perfectly clean to 

 ensure easy motion. If the tube does not move easily enough, rub it with a little almond 

 or watchmaker's oil, and move it upwards and downwards till it glides easily. Wipe off the 

 surplus oil with a cloth. 



(b~) The fine adjustment, is accomplished by rotating a milled head or screw placed at the 

 upper part of the pedestal of the microscope. It serves to bring the object accurately into 

 focus after its outline has been brought into view by means of the coarse adjustment. 



The stage, on which the object is placed, ought to be fixed, and at least three inches in 

 breadth. In Hartnack's microscope it is placed at a very convenient height above the table, 

 so that when the outer surface of the left hand is resting on the table, the thumb and fore- 



