246 Transactions of the 



III. — On the Use of Black Shadow MarTcings and on a Black 

 Shadoiv Illuminator. 



By Dr. Koyston-Pigott, M.A., F.E.S., &c. 



(Read before the Royal Miceoscopical Society, May 6, 1874.) 



The goodness of a telescope may be very fairly judged by the ap- 

 pearance of what is known to be black shadow markings. It is 

 only natural that the same method should be applied to objects 

 observed by the microscope. 



But it is impossible to produce these in all their beautiful inten- 

 sity either with a bad magnifier or a bad illuminator. 



The bad glass cannot show them if there, but the bad illumi- 

 nator cannot produce them. 



Cross rays cause as many shadows as there are directions. The 

 shadow of an object illuminated from different directions produces a 

 complex shadow of great intensity totally differing from the origi- 

 nal. Thus if you hold up your hand and throw shadows from 

 several candles, only the shadow common to each will be black on 

 white paper, and this will bear httle or no resemblance to the 

 original. 



It is known that parallel rays give the clearest shadows, but for 

 minute objects even a divergent pencil will give a finely formed 

 shadow ; or a convergent one provided there is only one origin of 

 light ; the penumbra is reduced as the size of the origin is dimi- 

 nished ; but the most convenient of all for black shadow illumina- 

 tions is the method of using an obliquely placed objective described 

 by me,* and now illustrated by the instrument sent for inspection 

 to be described farther on. 



Employing the other day some very old port wine as an immer- 

 sion fluid, I was surprised to see some extremely minute monads 

 rolhng and careering in the fluid more intensely defined than ever. 

 The black shadow illuminator produced in each an intensely black 

 appearance, which rendered their rotation and movements unusually 

 distinct. 



* Dec, 1869, ' M. M. J.' " Aplanatic pencil of rays, axis inclined from 15^ 

 to 20°." SoUitt had long before used obliquely placed condensing lenses, but 

 these were not aplmiatic. I have used this method at least ten years, employing 

 inclined object-glasses of various powers. The Ross IJ, a very fine glass 

 answers admirably. Messrs. Powell and Lealand in 1S62 constructed for me a 

 large semicircular arc attached by swivels undemeatli the stage so as to admit 

 illumination by extremely oblique rays. I then applied as condensers inverted 

 variously stopped off Huyghenian eye-pieces (achromatic, as generally supposed), 

 and next constructed a sub-stage within the main-stage carrying a gimbal with 

 rectangular motions, into which was introduced an old inch, and sometimes a 

 half-inch of Powell's ; these were adapted in 1863. I dure say the same idea has 

 occurred to and been executed by others. Some years after this Dr. Matthews, 

 I believe, advocated the same plan. (I showed this stage to Powell and Lealand 

 shortly after I had made it.) 



