72 



THE MICROSCOPE 



Finders. 



Vernier. 



Polarising 

 apparatus. 



•o 



to be at an inclination. It is marked for the correct position 

 for the use of the Beck vertical camera, or may be set at any 

 other position. 



A large amount of time is saved in examining specimens if 

 the position of a particular object or of a portion of a slide can 

 be recorded for future reference. For this reason mechanical 

 stages are provided with divided scales and verniers. The 

 readings of these scales are taken when the desired object is in 

 the centre of the field. These readings can be written on the 

 label on the slide, and the object in question can always be 

 found again by setting the stage so that the scales read these 

 numbers. 



For those who are not familiar with the use of a vernier, the 

 following description may be useful. The scales of the mechanical 

 stages are all divided in millimetres with a vernier which reads 

 to 1/10 of a mm. For a rough reading the first line with arrow- 

 head on the right-hand scale (Fig. 82) may be used 

 as an index, and the distance which it is beyond one 

 of the lines estimated, thus the reading of the scale 

 as shown in the figure would be about 13 J. For a 

 more accurate reading the other lines on the right- 

 hand scale, which form what is known as the vernier, 

 should be examined. The line with arrowhead is 

 not opposite any division on the long scale, but it 

 will be found that one of the lines on this scale 

 is opposite a division — in the case illustrated it is 

 the fourth line — this shows that the true reading 

 is not 13 J, but is 134. If it had been the eighth 

 line that was opposite a division, it would have 

 been 13-8, and so on. 



Polarising apparatus consists of a polarising (Nicol) prism in 

 a revolving fitting which pushes into the substage of the micro- 

 scope, a plate of selenite in a detachable tube sliding over the 

 polarising prism, and an analysing (Nicol) prism in a revolving 

 mount which screws into the nosepiece of the microscope between 

 the body of the microscope and the object glass. An analysing 

 prism in a special eyepiece may be used instead of the analyser 

 over the object glass if preferred. A polarising apparatus is 

 essential for the study of rocks, and is always supplied in petrologi- 

 cal microscopes ; but it is used on an ordinary microscope for the 

 study of crystals, starch, and many organic substances. A starch 

 granule can always be recognised by its means, as it shows under 

 polarised light a black or coloured cross, due to the crystalline 

 refraction of the material. Sugar and other crystals display 

 brilliant colours, and such materials as horses' hoofs, wax, or 

 finger-nails, show the structure in a manner that is not other- 

 wise seen. An explanation of the reason for the appear- 

 ances obtained with polarised light involve a full discussion 



Fia. 82.— 

 Vernier. 



