It very often occurs that the pencil point can on 

 no condition be seen distinctly, but this is usually 

 due to abnormal sight, with which persons are often 

 afBicted. In these cases, the glasses which are 

 required for reading should also be used in drawing. 

 The difficulty is not experienced in the image, as this 

 can be adjusted to the eye. 



Determining the Magnifying Power. Al- 

 though a magnifying table may be furnished, this 

 gives the powers merely approximately, as more or 

 less variation occurs in objectives and eye-pieces of 

 the same kind. As it is interesting, and sometimes 

 important, to know the exact magnifying power, a 

 simple method is mentioned. Procure a stage micro- 

 meter, divided into T i inch and TT5 Vo i ncn > an< 3 per- 

 haps ^oVo" inch, or, if prefered, any suitable division 

 in millimetre. Place the micrometer on the stage, 

 focus and incline the microscope, as if for drawing, 

 to within 10 inches from the table and attach the 

 camera lucida ; for low powers, T f J1F divisions may be 

 used ; for the higher ones TIJ Vo- or more ; the division 

 as now projected may be marked upon the paper and 

 then measured off with a rule divided into inches and 

 J^ inches ; if, for instance, the ^^ divisions are 

 used, and one division on the paper covered 1 inch 

 on the rule, it is evident that the magnifying power 

 is 1,000 times ; if it covered -f^, equal to , on the 

 rule, it would be 200 diameters, and so on. 



Measuring the Size of an Object. A simple 

 and reliable way of learning the size of an object is 



