The President's Address. By Prof. P. Martin Duncan. 173 



to care more for the instrument than most of them have done 

 hitherto. 



Most of the inexperienced, and not a few experienced Microscope- 

 possessors illumitiate, when transmitted light is used, in a manner 

 exactly opposite to that which they follow with the unaided eye. 

 They get all the light possible from a reflecting surface. Common 

 experience teaches us that there is an exact relation between the 

 possibility of seeing the half and lower tints and of searching the 

 depths of shadows, and the intensity of the hght entering the eye. 

 It tells us that only outlines are well shown as sky lines, or when 

 a brilliant light is passed around and through the body examined. 



Yet a pleasant evening with the Microscope generally means a 

 painful time for the eye. A good glare of light, thanks to lamp, 

 condenser, mirror, and forgotten diaphragm, appears to be almost 

 invariably a desideratum to the beginner. Experiences teaches, 

 however, and the advanced microscopist never uses more light than 

 is absolutely necessary, and increases and diminishes the illumuia- 

 tion during the careful observation of an object, not only by 

 employing a less intense source of Hght, but also by using 

 diaphragms of different sizes. 



The employment of different tints of coloured light, especially 

 pale blue and green, gives a wonderful rehef to the eye when 

 transmitted hght is used with or without a dark black ground. In 

 examining siliceous organisms the relief is great; but practice 

 follows theory in rejecting purples and aUied tints, the more deHcate 

 shades of which are not readily distinguished by the eye. One of 

 our Fellows has lately introduced a very simple apparatus which 

 enables a succession of colours or a group of tints to be used, and 

 there is no doubt that it will be much valued. 



The method of application of the electric light to the Micro- 

 scope, and the beautiful apparatus which has been exhibited before 

 the Society, must have impressed everybody that it will be the 

 light of the future. The brilliancy and coolness of the light, and 

 the possibility of directing it readily in investigating opaque objects 

 with low and very high powers, commend the method of illumina- 

 tion. It will be of great use in investigating objects by means of 

 high powers with reflectors within objectives, and in moving around 

 opaque objects which are well La focus under low powers, and whose 

 surfaces are difficult to define under ordinary circumstances in a 

 short time and with the usual appliances. We may expect that on 

 all occasions when there is an exhibition of a considerable number 

 of Microscopes, the electric light will be used, and the dangerous 

 and offensive rock-oils abolished. 



Since microscopy has been extended to the examination of 

 sections of rocks composed of different minerals, the truth that 

 some can be roughly distinguished by their dichroism under the 



