50 The Illumination of Objects. 



Collins probably did not know this, and his plan is better. 

 The " Graduating Diaphragm " fits under the stage like the 

 common revolving diaphragm, and a screw movement enables 

 the aperture to be reduced almost to a point, or opened to 

 considerable extent. By this means it is easy to find the exact 

 quantity of light that gives prominence to the most trans- 

 parent portions of an object, and then to add more light until 

 its more opaque parts transmit what they can. In the absence 

 of this instrument, which we strongly recommend all micro- 

 scopists to adopt, the gradations of light should be obtained 

 hj other means, such as using the ordinary diaphragm, turning 

 the lamp higher or lower, interposing screens of paper ren- 

 dered partially transparent by spermaceti, etc., etc. 



If after having gone through a series of experiments in 

 which light is allowed to reach the object straight, or at right 

 angles to its flat surface, and we have ascertained the effects of 

 carefully regulating the bulk of the illuminating beam and its 

 intensity, we next proceed to change the direction of the light, 

 the appearances will be modified : a slanting beam will pass 

 through thicknesses different from those which a straight beam 

 traverses. If the slant is sufficient to cause the rays of light, 

 in the absence of any object, to go away from our eye, and the 

 presence of an object allows us to see them, this effect can only 

 be produced by the refractive power of the object, or of certain 

 portions of it. If some parts do not refract the light so sent, 

 they will look dark, while the refracting portions will look light. 

 We must still pay great attention to the quantity of light 

 transmitted, and by varying the quantity we shall obtain 

 different effects. 



Slanting illumination of transparent objects may be one- 

 sided or all-sided, or something between the two. That is, 

 by means of various pieces of apparatus we may send a pencil 

 of light from one direction, or from two directions, or from all 

 sides at once, and the effects will vary in each case. Very 

 slight changes of direction in the light pencil will cause 

 noticeable differences in the aspect of delicate objects. For 

 example, if we employ an achromatic condenser, we shall find 

 that a slight change in the angle of the mirror, throwing the 

 light up through it, will cause the appearance of diatoms, or 

 delicate live objects, such as infusoria, to differ to the extent 

 that certain evidences of structure come and go, as the position 

 of the mirror is changed. 



Every object examined by the microscope has an appre- 

 ciable thickness, and we can easily perceive that its appearance 

 must vary according to the angle of which the light strikes 

 it. A very oblique light acts almost entirely on the surface, 

 and if properly directed and regulated, can give us surface, 



