COMPOUND MICEOSCOPE. 765 



In the handle of the first-mentioned disc there is a screw (fig. 945, 2, 

 d), which passes through it, and by the motion of which the two 

 handles can be separated or allowed to come close to each other. By 

 this means an exact focal distance can be obtained. A drop of fluid 

 containing Diatoms, or any minute object, is placed on the outside of 

 the thin glass in the silver ring, and it is theij covered by a similar 

 piece of thin glass, which adheres by means of the fluid. The object 

 being brought into focus, as in fig. 945, 2, the observer can distinguish 

 the characters of the microscopic plant, so as to determine whether it 

 is necessary to take specimens home for more careful examination by 

 the compound microscope. 



In the Compound microscope there are two sets of lenses — the one 

 called the object-glass or objective, the other the eye-piece or ocular. The 

 first receives the rays from the object, and bringing them to new foci, 

 forms an image, which the second treats as an original object, and mag- 

 nifies it just as the single microscope magnified the object itself. The 

 image is inverted, but this may be remedied by making the rays pass 

 through another set of lenses in the tube of the microscope, called an 

 erector. In the construction of the object-glasses, great care is taken 

 to render them achromatic. Those made by the most eminent Lon- 

 don makers consist of two or three compoimd lenses, which cannot be 

 used separately, but are fixed together in a tube. In the case of high 

 powers, the object-glasses are also provided with an adjustment for 

 the thickness of the glass covering the object to be viewed. This ad- 

 justment makes up for the refraction caused by the passage of light 

 through thin glass of different thickness, and is accomplished by 

 altering the distance between the outer and middle pairs of lenses in 

 the object-glass. This adaptation is especially necessary in the case 

 of a glass with a large angle of aperture. The eye-piece, also, must 

 be so formed as to be free from error. That used is called Huyghens', 

 and consists of two plano-convex lenses with their plane sides towards 

 the eye, and placed at a distance apart equal to half the sum of their 

 focal lengths, with a diaphragm inserted midway between the lenses. 

 In this eye-piece, the lens next the eye is called the eye-glass, the 

 other the field-glass. By the Huyghenian or negative eye-piece the 

 object is seen inverted. The Eamsden or positive eye-piece consists 

 of two plano-convex glasses, with the convex surfaces directed towards 

 each other ; by it objects are seen erect, and it is often used as a 

 micrometer eye-piece, that is, for measuring objects. The eye-pieces 

 supplied with the best microscopes are usually three, and they are so 

 constructed, that, with each of the object-glasses, they give a certain 

 amplification of the object, the powers being m the proportion of 1, 2, 

 and 3 or 1, 1^, and 2 J. In the best microscopes there is also an 

 achromatic condenser or eclairage, through which the light reflected 

 from the mirror passes. The amplification by means of an eye-piece 



