DR. MATTHEWS ON A NEW METHOD OF ILLUMINATION. 83 



The effect was that in the case of the spot stop a hollow cone of 

 rays resulted ; in others, oblique rays in one, two, or more, 

 azimuths. Certain markings in so-called test objects were thus 

 displayed, but no one knows to this hour, with any certainty, what 

 their nature is, simply because the light which should fall on them 

 empartially, if I may use the expression, is only used at the will of 

 the observer, in certain arbitrary directions. No wonder that some 

 one said to me, very recently, " You can make any appearances you 

 please with oblique illumination !" 



It seems to me practically absurd first to concentrate all the light 

 you can on an object, then to complain that there is so much, and 

 in such a wrong direction, that you cannot properly see it ; and 

 next to proceed to cut off Jths or f ths of that light in the hope 

 and expectation of seeing better what you want to see ! And yet 

 this is done every day, when we use a condenser with spots or stops 

 of any kind. In the spot stop, the case is more peculiarly 

 unreasonable, for then we get oblique rays certainly; but as 

 they are in every azimuth at once they must neutralise each, other 

 in a great degree. I am even of opinion that for this very reason 

 the means of centering used with condensers were a mistake, and 

 that they have probably worked at their best, when they were not 

 centered. 



Let us, therefore, go back to first principles. Let us consult 

 nature. 



Let me ask any astronomer when he sees the moon best ? What 

 is the nature of his difficulties in the observation of the planets ? 

 Is it not in the first case when she is not in the full, and do not his 

 difficulties in the second partly result from the opposite reason — 

 that they are always at the full, the source of light in both cases 

 remaining unalterable at his will ? I ask of any photographer 

 when he can get the best effects out of a landscape, when the sun 

 is behind his camera or at its side ? I think that I need not doubt of 

 the answer. And now comes the important question. How is 

 (what I think I may fairly call) this misapplication of that good 

 thing, the condenser, to be remedied ? I reply, not by interposing 

 stops or arbitrary openings of any kind in the path of the beam of 

 light, in order that by cutting off some of its rays, either radially 

 or centrally, the rest may be left oblique ; but by making the whole 

 bundle oblique. Let me explain in what way and by what agency. 



Mr. Ross lays it down as an axiom quoted by Mr. Hogg on the 



