164 THE MICROSCOPE : ITS 



used, the light of the microscope must be so regulated that it is of 

 an intensity as nearly as possible equal to that from the paper on 

 which the drawing is being made ; when the light from the paper 

 is more intense than that of the microscope, it is impossible to 

 distinguish the point of the pencil sufficiently well. In this case 

 the light from the paper must be reduced by means of a screw. 



Passing Book II., which treats briefly of simple microscopes, 

 and of projection — Solar, Gas, or Photo-electric — microscopes, we 

 come to Book III., which describes to the reader what should be 

 the situation and arrangement of the work-room ; choice of light; 

 hygienic rules for microscopical research, in which we are told 

 there is absolutely no foundation for the statement that micro- 

 scopical research is injurious to the eyes, and that no trouble need 

 be feared by anyone who will take heed of the following advice : 

 " I. — Do not make observations directly after a meal. 



2. — Let the field of the microscope be comfortably illuminated. 

 Always avoid brilliant illumination, and on no account use [direct] 

 solar light for ordinary observation. It is only during experiments 

 with polarised light, photo-micrography, and with monochromatic 

 light, that solar light can really be employed to advantage. 



3. — As soon as your eyes feel at all fatigued, suspend your 

 observations at once. This is of the greatest importance. 



4. — An excellent hygienic rule, which has greatly assisted us 

 during the last six years, is to wash the eyes thoroughly every 

 morning with warm water. We use a litre (if pints) of water 

 for this daily ablution. The warm water thus employed produces 

 at first a very slight congestion, followed immediately by an excel- 

 lent reaction. We cannot too strongly recommend this washing, 

 which rests the eyes. Cold water, on the other hand, gives a 

 momentary calm, followed afterwards by a congestion of the 

 visual organ." 



Descriptions, covering nearly 100 pages, are given of instru- 

 ments manufactured by various American, Continental, and English 

 opticians ; this portion of the work is very fully illustrated. Concise 

 instructions are given on Photo-Micrography, in relation to which 

 a variety of apparatus is illustrated; these are mostly of continental 

 manufacture. The causes of error in microscopical observations 

 are pointed out, one of them being Muscce volitantes^ popularly 



