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Some Suggestions on Oblique Illumination. By W. Hislop. 

 (Read Febmary 28th, 1868.J 



The effect of passing the illuminating ray through a transparent 

 medium of considerable refractive power, such as that which is pre- 

 sented by the ordinary glass slide, is not sufficiently considered by 

 microscopists, and hence leads to considerable misajiprehension, and 

 in the act of manipulation to great loss of time and uncertainty of 

 result. A moderate knowledge of the fundamental laws of optics 

 or mechanics will often prevent us from attempting that which is 

 impossible, as well as enable us to refer to their right causes the 

 failures which we meet with. Thus, a man who understands the 

 elementary principles of the science of Mechanics, and is practi- 

 cally acquainted with its details, even in a moderate degree, would 

 hardly fall into the too common error of seeking for a machine to 

 produce perpetual motion, or into the still more common one of en- 

 deavouring to originate force. And so with the somew^hat more 

 abstnise science of Optics. The law^s of refraction are fixed, and 

 we shall only succeed in our investigations when they are carried 

 out in subjection to these known and immutable laws. 



All objects which are permanently mounted so as to be viewed 

 by transmitted light, are placed upon a plate composed of some 

 transparent medium which is almost exclusively composed of glass. 

 The means of illumination are various ; direct light, or light re- 

 flected from the surface of a mirror, or through a prism, are 

 severally used according to the fancy or means of the operator. 

 This light is again often modified by pieces of apparatus called 

 condensers, which, by their variety, complication, and consequent 

 difficulty of manipulation, frequently bewilder the earnest student. 

 But in all these cases we must remember that the ray of light, after 

 it leaves our apparatus, whether that apparatus be simple or com- 

 plex, does not reach our object without suffering a change, if that 

 object be, as usual, supported on a plate of glass. Let us now 

 suppose that we are examining — say, a frustule of Pleurosigma 

 angulatum in the usual way. If it w'ere possible to obtain a beam 

 of light of which the component rays should be perfectly parallel, 

 and if this parallelism could be so kept through object, objective, 

 and eyepiece into the eye, we should probably observe the object 



