THE MICROSCOPE. 13 



ACCESSORIES. 



The sub-stage. This is an attachment supported beneath the 

 stage and is designed to receive the iris diaphragm, Abbe condenser, 

 .etc. 



Iris diaphragm. This is supported by the sub-stage and is so 

 constructed that the aperture for admitting light may be regulated 

 by turning a milled-head. 



Abbe condenser. This is an apparatus containing a lens of very 

 short focus, and is capable of producing intense illumination. It 

 is supported by the sub-stage. 



Mechanical stage. This is attached to the microscope so as to 

 work above the stage, and is designed to hold the slide and so change 

 its position as to bring every part of the object into the field of view. 



Camera Lucida. This is an apparatus which is attached to the 

 tube of the microscope, and is designed to assist in making diagrams 

 of the object .studied. 



Polariscope. Polarization consits in reducing vibrations of light 

 to one plane. This is accomplished by the polariscope, which con- 

 sists of a polarizer and an analyzer. The polarizer consists of a 

 crystal of Iceland spar, which by double refraction separates the or- 

 dinary from the extraordinary ray. The analyzer generally used is a 

 Nicol's prism, which consists of a crystal of Iceland spar split diag- 

 onally and the pieces then cemented together with Canada balsam. 

 The Canada balsam produces the total reflection of the ordinary ray, 

 while the extraordinary ray passes through. The analyzer is used 

 to detect polarized light. 



Micrometer. There are two kinds ofmicrometers the stage mi- 

 crometer and the eye-piece micrometer. The stage micrometer is 

 a small glass slide, upon which is a graduated scale, the graduations 

 being in millimeters and tenths of a millimeter. It is used in de- 

 termining the magnifying power of the microscope. 



Laboratory exercise No. 2. To obtain a focus. Place upon the stag-e 

 a prepared slide; adjust the reflector so as to illuminate the object; 

 using- first the low power, by means of the coarse-adjustment lower 

 the objective until it is below the focal point. If the objective be a 

 two-thirds inch, the focal point will be somewhat less than two-thirds 

 of an inch from the object. Now, with the eye at the eye-piece work 

 upward until the object appears in view^ Perfect the focusing- by using- 



