MICROSCOPICAL EXAMINATION OF COMMON OBJECTS. 7 



in the diagram), which serves to concentrate the light thrown up by the 

 mirror to the centre of the object, is valuable when high powers and stained 

 preparations are employed. 



The combinations of objectives ( inch and J inch focal distance) and oculars 

 above referred to will generally give a magnifying power of from 50 to 400 

 diameters, and this is sufficient for most purposes of histology. But to bring 

 out minute points of detail in the structure of cells and of certain tissues- 

 examination with much higher magnifying powers may be necessary. 

 Objectives of high power are usually made as immersion-lenses ; i.e., they 

 are constructed to form a proper image of the object when the lowermost lens 

 of the system is immersed in a layer of liquid which lies on the cover-glass of 

 the object and has a refractive index not far removed from that of the glass 

 itself. For this purpose either water, or an essential oil (e.g. oil of cedar) is 

 used. Many advantages are obtained by the employment of these lenses, 

 especially those for oil-immersion. 



The best lenses for histological work are those which are made of the 

 so-called ' apochromatic ' glass of Zeiss ; with these, specially constructed 

 'compensating' eye-pieces are used. The only obstacle to their general use is 

 their price. 



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 YO an< ^ 100 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 re- 

 presentation is thus obtained of the micrometer-scale. Mark upon it the num- 

 ber 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 y^ 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. The lines on the English stage-micrometers are usually ruled y^ 

 and YO^OU i llc h apart. 1 



Before beginning the study of histology the student should endeavour to 

 familiarise himself with the use of the microscope, and at the same time 

 learn to recognise some of the chief objects which are liable to occur 

 accidentally in microscopic specimens. On this account it has been considered 

 desirable to introduce directions for the examination of starch-granules, 

 moulds and torulse, air-bubbles, linen, cotton, and woollen fibres, and the 

 usual constituents of the dust of a room, into the first practical lesson. 



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. 6, 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 



1 For the method of measuring with an ocular micrometer, and for determining 

 the magnifying power of a microscope, the student is referred to the author's Course of 

 Practical Histology. 



