506 Transactions of the Society. 



required to be more than three inches from the object, the details 

 of which are best shown when the rays from the mirror fall upon it 

 before crossing, and the centre (of mirror) should be, especially by 

 lamplight, in the axis of the IMicroscope. For obscure objects 

 seen by transmitted light and /or outline a full central illumination 

 is commonly best ; tjut for seeing delicate lines (e. g. on insect 

 scales) it should be made to fall obliquely and in a direction at 

 right angles to the lines to be viewed. The diaphragm is often of 

 great use in modifying the light and stopping such rays as would 

 confuse the image (especially with low or moderate powers), but 

 many cases occur when the effects desired are best produced by 

 admitting the whole from the mirror. If an achromatic condenser 

 is employed instead of the diaphragm its axis should correspond 

 with that of the body, and its glasses when adjusted to their right 

 place should show the image of the source of artificial light, or by 

 day, that of a cloud or window-bar in the field of the Microscope 

 while the object to be viewed is in focus." 



The above directions are quite compatible with a correct theory 

 of illumination, though it cannot be said that they constitute any 

 systematic expression of such a theory. If we now turn to the 

 practical directions given by Sir D. Brewster in his article on 

 the Microscope in the seventh edition of the 'Encyclopaedia 

 Britannica,' * we find certain directions relating to the position 

 of the observer, the condition of the conjunctiva of the eye, the 

 proper direction of lined objects in relation to the flow of moisture 

 over the conjunctiva, which may be passed over, and certain rules 

 to be observed in managing the illumination, which I here quote. 



Firstly. The eye should be protected from all extraneous light, 

 and should not receive any of the light which proceeds from the 

 illuminating centre except that portion of it which is transmitted 

 through or reflected from the object. 



Fifthly. The field of view should be contracted so as to exclude 

 every part of the object excepting that which is under immediate 

 examination. 



Sixthly. The hght employed for illuminating the object 

 should have as small a diameter as possible. In the daytime it 

 should be a single hole in the window-shutter of a darkened room, 

 and at night it should be an aperture placed before an argand 

 lamp. 



Seventhly. In all cases, and particularly when very high 

 powers are requisite, the natural diameter of the light employed 

 should be diminished and its intensity increased by optical con- 

 trivances. 



The eiyhth direction refers to the use of homogeneous light. 



Sir D. Brewster's essay on the Microscope marks an important 



* Vol. XV. )S42. 



