122 SELECTION AND USE 



and watch the different appearances which are produced by 

 a change in the mode of illumination. Swinging the mirror to 

 one side, so as to send the light through the object in an 

 oblique direction, or, where the mirror cannot be turned to 

 one side, merely turning it on the trunnions which support it, 

 will often produce most important effects. 



From what has previously been said in regard to the neces- 

 sity for clear and brilliant sources "of illumination, the young 

 microscopist may, perhaps, be led to suppose that the field of 

 view cannot be too brilliantly illuminated. Such, however, is 

 far from being the case. With ordinary powers (those below 

 500 diameters) it is almost always necessary to moderate the 

 light, even of a flat-wicked lamp, and still more that of a 

 students' lamp. The finer details of an object cannot possibly 

 be made out if the illumination be too strong; they are 

 " drowned out," and the whole object becomes what artists and 

 engravers call flat. The light may be regulated by the dia- 

 phragm which has been previously described. Where the mi- 

 croscope is not furnished with a diaphragm, increasing the dis- 

 tance of the lamp from the instrument is the best mode of 

 lessening the intensity of the light. 



Very bright light is exceedingly trying to the eyes, and 

 therefore the student will find it advantageous to use lights of 

 moderate intensity, and to increase their efficiency in every 

 possible way. This may be done to a very great extent by 

 judicious management chiefly by excluding from the eye all 

 unnecessary light. In a room very brilliantly lighted with a 

 number of powerful argand burners, it would be impossible to 

 secure the proper illumination of a microscopic object by 

 means of a candle, for the eye, accustomed to the bright light, 

 would fail to be impressed by the weaker one. Extinguish the 

 bright lights, give the eye a short time for rest, and the candle 

 will answer very well. The principle thus illustrated finds a 

 practical application in the use of pasteboard shades surround- 

 ing the eye-piece, and excluding from the eye all light except 

 that which passes through the microscope. Such a shade is 

 easily made and adapted to any microscope, and is of great 

 service. We have also in our own practice carried out the 

 same principle by means of extra diaphragms to our eye- 



