OF DRAWING MICROSCOPIC OBJECTS. 



137 



secure only the outlines of the macroscopic picture, and then, when 

 this has been accompUshed with only moderate artistic dexterity, 

 ingenuity, and taste, one can readily reproduce a fac-simile or 

 likeness ; but where pure freehand work is required— such as we 

 observe in some of the ancient oil paintings, e.g., those of Rubens, 

 Rembrandt, Raphael, and others— the operator must rely entirely 

 upon his own artistic skill and taste. By repeated measurements 

 one will be able to obtain accurate and artistic results, such as we 

 now see in some of the most approved, artistic oil paintings, 

 water-colours, crayons, etc. 



Fig. 21. — Nachet's Grapho- Prism. 



In the freehand production of a microscopical drawing one 

 must be skilled in drawing. The necessity of this exceptional 

 artistic skill has led some of the expert microscopic artists to 

 devise an instrument which greatly facilitates the work and renders 

 it comparatively easy to any microscopist. This valuable instru- 

 ment is the grapho-prism. In drawing a picture from a specimen 

 under the microscope, all depends upon the optical measurements. 

 If the object be so small that the combined compound microscope 

 is brought into use, the optical measurements are easily and simply 

 procured by means of a micrometer-eyepiece; but if the object be 

 an extremely minute one (such as germs, baciUi, microbes, etc.), 

 other means must be employed to satisfy this necessity, and opti- 

 cians have therefore manufactured micrometer objectives which, of 

 course, are divided into the one-thousandth part of a millimetre : 

 but with the oil-immersions the lines of the graduated millimetre- 

 scale are invisible, therefore inapplicable. 



