14 Transactions of the Society. 



source of light having been duly centred, it will be found that 

 these two images afford an easier method of centring the substage 

 mechanism than that already described of observations made upon 

 the Fresnel rings. Thus, the iris- diaphragm being opened wide in 

 order to expose the full aperture of the condenser, we ought, when 

 the source of light is focused in the globule, to see it truly central. 

 If that is the case, the optical axis of the substage condenser is 

 coincident with the optical axis of the Microscope. Then a very 

 minute image of the globule itself will be seen occupying the exact 

 centre of the small image of the source of light. The slightest 

 displacement of the condenser disturbs this arrangement and 

 throws this opaque image of the globule visibly away from the 

 centre of the source of light. This, therefore, is an extremely 

 critical test of the centring of the condenser. The condenser 

 having been centred, you may now alter its focal position so as to 

 bring the aperture of the condenser and the image of the iris into 

 view in the globule. If now the iris be closed, its image will be 

 seen closing either truly upon the image of the globule or upon 

 some excentric point according as the iris is in or out of centre. 

 Here, again, the necessary adjustments are easily made, since their 

 progress can be followed with the eye. 



There is among the exhibits upon the table this evening one 

 which very strikingly indicates the great abundance in which light 

 comes back from the reflecting surfaces of the optical system. A 

 comparatively large globule, actually of diameter of -^ in., is mounted 

 under a ^-in. objective. The light is so arranged that a strong image 

 is thrown back from the observer's cornea, when his eye is placed 

 accurately at the eye-point of the instrument. The flashing of this 

 image across the centre of the globule forms a very striking object, 

 and it may be observed that in this experiment a very perfect 

 image of the globule is in this way formed, and may be momentarily 

 seen. But it is, of course, impossible to hold the eye stationary 

 enough for anything more than a flash view of this image. Beside 

 the corneal image a coloured and imperfectly focused image 

 reflected from the interior of the eye may also be seen. I mention 

 it not as an object upon which I have any observations to offer, 

 but for the purpose of drawing the attention of others to it who 

 will be able to study it to better purpose than I can. 



The phenomenon just described may be made the starting point 

 of an almost ideal test for resolving power. In place of the eye, 

 which is a moving object, we may substitute a mercury globule 

 properly mounted at the eye-point of the microscope. It will then 

 reflect light precisely as did the observer's cornea in the last 

 experiment, and if for this purpose we use a small mercury globule 

 (one having a diameter of j^ g of an inch is very suitable), it will 

 not impair the observer's view of the stage, when he looks down 

 the instrument. In that case he will see, not the image reflected 



