82 DR. MATTHEWS ON A NEW METHOD OF ILLUMINATION. 



many who hear me, as well as, I confess, to myself. And here it 

 is the place and now the time to aver that economy in apparatus 

 has ever been a prime motive in my experiments, provided that 

 efficiency and accuracy were not thereby impaired. In this I think 

 that I have succeeded almost beyond my most sanguine expecta- 

 tions. 



I may just observe, by the way, that we have heard very much 

 lately of the results of some other mode of illumination, of which 

 the means have not as yet been published. This is to be deplored, 

 as secresy is the enemy of true science, and surely that secresy is 

 unworthy which, while proclaiming results, does not indicate 

 means ! 



The great and important question now presents itself of the real 

 and true meaning of the word definition, since it is that, in com- 

 bination with resolving power, at which scientific microscopy 

 should aim. It means, I conceive, to put it as tersely as I can, 

 " The power of correct appreciation of light and shade in reference 

 to form, structure, and colour." And its completeness depends 

 mainly upon the angle at which light is incident on, or passes through 

 an object, as well as upon the amount and quality of that light. 



It was speedily found, as the microscope approached perfection, 

 that axial light did not fulfil all the conditions necessary for de- 

 finition, and so men, almost instinctively, turned the mirror out 

 of the axis, in order to get the effect of oblique illumination, find- 

 ing that more details were thus secured. Just on this principle, 

 astronomers observe the moon in her phases instead of at the full, 

 knowing that in the former case her surface has more of appreci- 

 able detail. 



This was perfectly easy as long as the mirror or the prism in any 

 form were employed. But when the condenser was substituted, 

 the conditions altered — the light once more became axial, and then 

 " definition" was impaired, if it were only from the great increase 

 of light incident in such a direction that it resembled the examina- 

 tion of the moon at her full, instead of in one of her phases. To 

 meet this, certain contrivances were employed called " stops," con- 

 sisting of a diaphragm so mounted excentrically as to present 

 certain variously-shaped openings in the axis of illumination. 

 Some of these were " spots," some mere slits or " slots " in one 

 azimuth ; others slots in tw # o directions, so as to divide and admit 

 light in two or more sets of rays. 



