DRAWING APPLIANCES. 43 



with the aid of our camera, and obtain thus, if the inclination of 

 the drawing-desk is correct, a figure which is likewise a circle ; 

 if, on the other hand, we have an ellipse, the slope of the draw- 

 ing-desk is not correct, and must be varied until a circle is 

 produced. Or, we set in position, and with a high power, the 

 stage micrometer recommended in the Introduction, i.e., a milli- 

 meter divided into 100 parts, engraved upon an object-slide. 

 We now turn the stage micrometer round through 90, so that 

 the engraved lines shall run from side to side, and succeed one 

 another fore and aft. In case the stage is too small to permit 

 such a position of the stage micrometer, we must change the 

 position of the microscope itself 90. The turning of the micro- 

 scope naturally renders necessary a readjustment of the mirror. 

 If our instrument is provided with a " concentric rotating stage," 

 or similar appliance, then it is only necessary to turn this ; such 

 a stage is very useful for drawing, as it enables us to place the 

 object in the desired position. If we have given the micrometer 

 its proper position, then, with the help of the camera, we draw 

 its lines upon the paper on the drawing-desk. The lines follow 

 one another up the slope of the desk. We shall succeed, 

 without much difficulty, in reproducing it exactly: but, as the 

 lines have a certain thickness, it is necessary that we should 

 represent always a definite edge of the line. The inclination 

 of the drawing- desk is correct when the distance apart of the 

 lines remains the same at all heights. If this distance increases 

 upwards on the desk, the slope must be made steeper ; if it 

 sinks, the slope should be lessened. As, however, small 

 errors in our measuring scale are not impossible, it is neces- 

 sary to represent several parts of it in the same way. In 

 this way we shall find that our desk should have a slope of 

 about 25. Having once found the correct slope, it is well to 

 have a desk made with its two supporting sides of the correct 

 heights. 



When we have obtained the correct inclination of the drawing- 

 desk, this figure, taken from the stage micrometer, can also be 

 used in order to calculate the magnification of the drawing, i.e., 

 the magnifying power of the combination of objective and eye- 

 piece in use. We know already that the lines which we have 

 drawn are O'Ol millimeter (i.e., approximately ^TW mcn ) apart. If 

 we find that now they lie say 2-4 mm. (i.e., nearly y 1 ^ inch) apart, 

 we know that the drawing is enlarged 240 times. This method is 



