GLASGOW SOCIETY OF FIELD NATURALISTS. 133 



made to revolve with it if a milled head is turned. The size of 

 the angle is read off on a brass plate which is divided into 360°. 



11. The erector or erecting glass, for showing the inverted image 

 in the same position as the object, is useful when a dissection is to 

 be made under the microscope. It consists of a tube carrying 

 three lenses, which is fixed into the lower end of the draw -tube, 

 and by its means the image can be more or less magnified. 



12. The centring glass, fitting over the eye-piece, serves for 

 testing the centricity of the illuminating apparatus and of the 

 sub-stage. It has a minute aperture, and by looking through it 

 both the diaphragms of the object-glass and of the condenser are 

 seen, which must appear concentric if the sub-stage is properly 

 centred. 



13. The object-finder, for finding objects on slides previously 

 seen, and the position of which has been i-egistered. I will only 

 explain Mr. Maltwood's finder, which consists of a small plate of 

 glass on which a field of one square inch is photographed. This 

 field is divided by horizontal and vertical lines into 2500 small 

 squares, each of which bears a different number. If the position 

 of an object under the microscope is to be registered, the Malt wood 

 finder is placed in exactly the same position as the slide, which is 

 resting against the ledge of the stage, and against a pin fixed on 

 it. The number of the corresponding field is then read off and 

 either marked on the label of the slide or in the catalogue. By 

 reversing this proceeding the registered object is easily found again. 



I have now arrived at the last chapter of my lecture, in which 

 I shall speak of microscopic illumination and illuminating 

 apparatus. 



Illumination in microscopy is the principal thing, and successful 

 observation, especially under high powers, is mainly dependent 

 upon proper illumination, for by its means inferior object-glasses 

 may be made to show more than the best objectives will show if 

 the object under examination is not properly illumined. The first 

 thing requisite is good light, and the light best suited for magnifi- 

 cation, up to 2000 diameters, is daylight refiected by a white cloud, 

 against which the sun shines. Such light is, at the same time, 

 least detrimental to the eye-sight. For the highest powers, and for 

 the resolution of the most difficult tests, sunlight may be employed, 

 but, owing to its great brilliancy, it should be modified by the 

 interpolation of a light modifier of roughly ground white glass, or it 



