Chap, xxvi ii.] IMMERSION LENSES. 363 



thus able to pass through it ; while, in the latter case, 

 the difference between the refractive index of glass 

 and air is so much greater that the rays would fall 011 

 the objective more obliquely, more would be unable 

 to pass through, and loss of light would result. 

 Instead of water, glycerine may be used. Oil of cedar 

 wood has been found specially useful by Prof. Abbe 

 of Jena, because its refractive and dispersive powers 

 are nearly that of glass. Lenses made for use in this 

 way are called immersion lenses, but it is usually 

 only for very high powers that they are employed. 



Mechanical parts of a compound micro- 

 scope. Pig. 163 represents a compound microscope 

 of Zeiss's model. It consists of a firm foot which sup- 

 ports an upright stand. The stand is jointed so as to 

 permit of the microscope being inclined or placed 

 horizontally. From the stand projects a horizontal 

 arm p, terminating in a ring r, in which is screwed a 

 tube T. This tube is split so as to permit the lens 

 tube ti, to slide up and down easily. The lens tube 

 consists of an outer tube t, movable up and down in 

 the split tube T,, by means of the milled edge m'. 

 Fitting into the tube t, and also movable in it, is a 

 second tube D, which is called the draw-tube, and is 

 pushed home into t, or drawn out, by the milled edge 

 m. E points to the outer end of the eye-piece which 

 fits into D. At the other end L of the microscope 

 tube is a screw adjustment which permits of the lenses 

 being screwed on or off the tube, s is the stage on 

 which the object to be examined is laid, and on it are 

 two little spring slips for holding down the slide on 

 which the object lies. In the centre of the stage is 

 pierced an opening through which light can be directed 

 by the mirror M, placed a little distance under the 

 stage and movable in all directions. Under the stage 

 is a disc pierced with openings of various sizes, the 

 smallest no larger than a pin-head, any one of which 



