The Illumination of Objects. 51 



and often superficial information. If we want, after haying 

 seen the surface, to look below it, we must not allow any rays 

 of too much obliquity to fall upon the object ; but try various 

 changes, between light sent right through it at right angles to 

 its flat surface, and light transmitted with moderate degrees of 

 obliquity. In such experiments the size of the illuminating 

 pencil and its intensity both require adjustment, and although 

 a certain best position or arrangement may be obtained, others 

 that are on the whole worse may yet give special information 

 necessary for the right understanding of the object. 



In illuminating transparent objects, we can employ the 

 simple mirror, the mirror with diaphragm, the achromatic 

 condenser, or similar contrivances ; but in all cases we are 

 regulating direction and quantity of light, and nothing more. 

 If we stop out the marginal rays of a pencil of light emerg- 

 ing from a lens, we are simply determining that no rays shall 

 reach the object but such as have little divergence. If we 

 stop out the central rays, and allow only the marginal ones to 

 be transmitted, we are merely providing that the object shall 

 only be lit by rays of greater or less divergence, and not by 

 any rays that are parallel, or have a small divergence. 



With objects of considerable refractive power, we may 

 employ rays so oblique that they would escape us entirely if 

 they were not bent up towards our eyes. The spot lens and 

 the parabolic illumination thus give a dark ground illumination 

 with appropriate objects. They succeed beautifully with 

 objects of sufficient refractive power, and they fail with those 

 that do not possess it in the requisite degree. When objects 

 contain two substances, or the same substances in two forms, in 

 the one case highly refractive, and in the other highly trans- 

 missive and freely refractive, the illumination that displays the 

 one to advantage will not show the other, and any reasoning 

 founded upon one view only will be unsound. In like manner, 

 if two parts differ much in transparency, one illumination 

 cannot be equally advantageous for both. 



In displaying objects to our friends we should adopt a mode 

 of illumination that is generally accommodated to their pecu- 

 liarities, but in studying them by ourselves we should ring the 

 changes upon all the modes of illumination of which they are 

 susceptible. 



The achromatic condenser, with its variety of stops, has 

 hitherto been the most important instrument for transparent 

 illumination ; but recently Mr. Highley has introduced an 

 invention of Mr. Webster that may wholly or partially super- 

 sede its use. This apparatus consists of a large achromatic 

 combination, with a plano-convex lens in front of it. It is fur- 

 nished with stops somewhat like the ordinary achromatic con- 



