6 THE ESSENTIALS OF HISTOLOGY 



1. Examination of starch-granules. Gently scrape the cut surface of a 

 potato with the point of a knife ; shake the starch-granules so obtained into 

 a drop of water upon a clean slide and apply a cover-glass. 



With the low power the starch-granules look like dark specks differing 

 considerably in size ; under the high power they are clear, flat, ovoid particles 

 (fig. 3, St), with a sharp outline when exactly focussed. Notice the change in 

 appearance of the outline as the microscope is focussed up or down. On close 

 examination fine concentric lines are to be seen in the granules arranged 

 around a minute spot which is generally placed eccentrically near the smaller 

 end of the granule. Sketch two or three starch -granules. 



Notice the appearance of air-bubbles in the water. If comparatively 

 large they are clear in the middle, with a broad dark border due to refraction 

 of the light ; if small they may look entirely dark. 



2. Examine fibres of linen and of cotton in water, using a high power. 

 Compare the well-defined, relatively coarse, striated, and slightly twisted linen- 

 with the longer, finer, and more twisted cotton-fibres. Sketch one of each 

 kind. 



3. Mount two or three hairs from the head in water and look at them, 

 first with the low, then with the high power. Examine also some fibres 

 from any woollen material and compare them with the hairs. They have 

 the same structure, although the wool is finer and is curled-; its structure 

 may be partly obscured by the dye. Draw one or two woollen fibres. 



4. Examine some dust of the room in water with a high power. In 

 addition to numerous groups of black particles of carbon (soot) there will 

 probably be seen fibres of linen, cotton, or wool, and shed epithelium-cells 

 derived from the epidermis. 



5. Prepare a scale to serve for measuring objects under the microscope. To 

 do this put a stage-micrometer (which is a glass slide ruled in the centre, with 

 the lines ^ and j! 5 millimeter apart) under the microscope in such a manner 

 that the lines run from left to right (the microscope must not be inclined). 

 Focus them exactly. Put a piece of white card on the table at the right of 

 the microscope. Look through the instrument with the left eye, keeping the 

 right eye open. The lines of the micrometer will appear projected upon the 

 paper. Mark their apparent distance with pencil upon the card, and after- 

 wards make a scale of lines in ink the same interval apart. A magnified repre- 

 sentation is thus obtained of the micrometer scale. Mark upon it the number 

 of the eye-piece and of the objective, and the length of the microscope tube. 

 This scale-card will serve for the measurement of any object without the 

 further use of the micrometer. To measure an object, place the scale-card 

 upon the table to the right of the microscope and view the object with the 

 left eye, keeping the right eye open. The object appears projected upon 

 the scale, and its size in ~ or yj^ of a millimeter can be read off. It is 

 important that the same objective and eye-piece should be employed as were 

 used in making the scale, and that the microscope tube should be of the same 

 length. 



