Fia. 72. — 

 No. 3279, 

 Ruled Eye- 

 piece Plate. 



CHAPTEE IV 



SUNDRY APPARATUS 



The diawins of specimens seen under the microscope by free- Drawing 



hand suffers from the disadvantage that it is difficult to obtain specimens. 



accuracy in ^dimensions and relative proportions. Microscope 



drawings are seldom required as works of art, but must be 



accurate. ^ The simplest aid to accurate drawing is paper ruled Euied 



with lines in squares used in combination with a glass plate ruled °i"*™- 



into squares dropped into the eyepiece of the microscope. If 



the top lens of the eyepiece be unscrewed, it will 



be seen that about half-way down the tube there 



is a stop; a ruled plate (Fig. 72) can be dropped 



upon this stop, when it will be foimd to be in the 



focus of the top lens. If the lines are not quite 



distinct when the top lens is screwed home, the 



latter may be slightly unscrewed till the lines come 



sharply into focus. 



This method of drawing is popular because the 

 position of the main outlines and salient features 

 of an object can be accurately ascertained, and as much of 

 the detail as is desired filled in freehand. A sketch showing the 

 points of importance, leaving out much of the extraneous detail, 

 is sometimes of more scientific value than a photograph, which 

 shows so much detail that it is difficult to pick out the features 

 of special interest. 



Objects sketched in this manner may be measured by reference Measuring 

 to a stage micrometer. This is a 3 X 1-inch glass sUp with lines sp*"""^'^- 

 ruled on it 1/10 and 1/100 of a millimetre, or 1/100 and 1/1000 

 of an inch ; and if it be placed on the stage of the microscope 

 and viewed under the same conditions as the object that has been 

 drawn by means of the squared paper, it is easy to see how many 

 1/lOOths of a millimetre or 1/lOOOths of an inch are included in 

 each square. This can be noted on the paper, and the dimensions 

 of the object may be obtained by measuring the drawing. 



A glass plate 4x1 inches, with divisions etched on its lower 

 surface, is the most convenient scale for making such measure- 

 ments on the drawing. 



The measiurement can also be made without making a drawing, 

 for once the value of a square in himdredths of a millimetre or 



67 



