MICROSCOPIC ILLUSTRATIONS. 259 



colour and form ; it renders every opaque object a mass of 

 confusion. 



17. There are a variety of optical deceptions produced 

 by microscopes and engiscopes, against which observers 

 cannot be too strongly guarded, as inadvertent persons 

 have brought great disgrace on microscopic science by 

 trusting too much to the testimony of their instruments. 

 It requires long and repeated observation to enable us to 

 be quite certain of the nature of what we see: for 1st, 

 we are seldom or never in the habit of viewing common 

 objects by intercepted light; vision by it is therefore 

 altogether new to us, and the phenomena presented by 

 bodies subjected to its influence are totally different to 

 what they assume as opaque bodies, which is the usual 

 way in which we see common objects; 2dly, all micro- 

 scopic vision may be considered as accomplished under 

 very large angles, which is again a totally new .way of 

 seeing to the uninitiated. The object is in a manner 

 placed in a most unnatural state of proximity to the eye, 

 and consequently assumes a totally different character to 

 that produced by objects at our common visual distance, 

 which must inevitably perplex us till we become habituated 

 to it. There is one general law to which all transparent 

 objects are subjected ; namely, that those parts which 

 appear brightest and clearest in them, are almost inva- 

 riably the thinnest; a want of transparency nearly in every 

 case argues thickness and substance. Much may be done 

 towards their verification by examining sections of them as 

 opaque objects,* where this is practicable ; and care should 



* EXAMPLE. In examining the eye of a libellula, which had been long 

 kept, with the light of a taper behind the stage, I was much surprised to find 

 that its lenses gave erect images of the candle instead of inverted ones. I 



