398 Transactions of the Society. 



XV. — On the Illumination of Objects under the Higher Powers 

 of the Microscope. 



By James Smith, F.R.M.S. 



{Bead 10th March, 1880.) 



Various ingenious appliances have from time to time been brought 

 under the notice of microscopists, having for their aim the better 

 iUuminatiou of objects vmder the higher powers of the Microscope. 

 I venture to lay before the Society the following method of illu- 

 mination, -which I have successfully carried out, and which will, I 

 think, commend itself as at once simple and effective, and ready to 

 the hand of every possessor of a Microscope. 



The method is simply a mode of using the bull's-eye condenser, 

 enabling it to be employed under circumstances which prevent 

 its efiiective use in the ordinary way, and is as follows : — 



The Microscope is placed in position for observation, with 

 the lamp in front (or at the side, as may be most convenient), about 

 3 inches off the flame, which should be somewhat lower than 

 the stage, and turned edgewise to the Microscope. The bull's- 

 eye condenser is then placed between the stage and the lamp, with 

 the plane side uppermost, the convex surface being a little higher 

 than the stage. The light strikes the plane surface of the 

 condenser, and is again reflected at a very obhque angle upon the 

 object on the stage, a sharp and brilliant wedge of light being 

 cast upon the slide. This is better seen, and the adjustment of 

 lamp and condenser more readily made, by placing in the first 

 instance a small shp of white card on the stage ; the object may 

 then be placed in position, and the objective focussed upon 

 it, and the final adjustment of the illuminator made while looMng 

 through the Microscope at the object. 



With respect to the powers used, I have worked with 

 excellent effect upon scales of Lepidoptera with the ^ and i inch 

 objectives, and some of the Diatomace^e (Pleurosigma), both 

 dry and in balsam, are beautifully shown, as are also scales of 

 Podura ; upon these latter the ^^-Jiich (dry) has been used most 

 successfully. With the tV immersion, however, I have obtained 

 the finest results with a powder varying from 1100 to 2000 

 diameters. I have worked upon Or slide of Pleurosigma (in balsam), 

 and such diatoms as P. formosiim, elonr/atum, hippocampus, 

 &c., are showai finely illuminated on a dark ground. It is 

 scarcely necessary to add that with these high powers a careful 

 manipulation of the light is necessary to bring out the best effect, 



