316 
METHODS IN PLANT HISTOLOGY 
be measured with far greater rapidity than by the other method. 
Upon a piece of cardboard, about as thick as a postal card, draw a 
series of scales like those shown in Figure 117. 
Make a scale for each objective. It is not necessary to make 
scales for all the oculars, but only for the one in most constant use. 
It is absolutely necessary to note the tube length, length of the bar 
of the camera mirror and inclination of the camera mirror, and the 
level at which the scale is made. A variation in any of these details 
will change the scale. 
In using the stage micrometer, place the cardboard on the table, 
and with the aid of the camera lucida sketch the rulings of the microm¬ 
eter. In Figure 117 note, for example, the scale drawn with Spencer 
16 mm. objective, ocular X6. The spaces are drawn from the tenths 
of a millimeter rulings of the stage micrometer. Therefore each space 
on the card represents one-tenth of a millimeter or 100 n, and the 
ten spaces shown on the card represent 1 mm., or 1,000 /*. By measur¬ 
ing with a metric rule the ten spaces upon the card, it is found that the 
scale is 102 mm. in length. The magnification of any drawing made 
with the same ocular and objective, under the same conditions, will 
therefore be 102 diameters. This does not mean that the magnifying 
power is 102 diameters, for the magnification of this combination is 
much less. A scale drawn at the level of the stage would show more 
nearly the magnifying power of the combination, but would still give 
too large a figure. The exact size of any object which has been 
sketched with this combination can now be measured by applying 
the cardboard scale, just as one would measure gross objects with a 
rule. 
The diameter of the field with this-combination is 1,700 ju* By 
knowing the diameter of the field with the various combinations, one 
can guess approximately the size of objects. 
Other combinations are made in the same way. An excellent 
check on the accuracy of the computations is to measure the same 
object by means of the ocular micrometer and by the scale card. If 
the results are the same, the computations are correct. 
In making sketches, it is a good plan to add the data which would 
be needed at any time in making measurements; e.g., Spencer objec¬ 
tive 16 mm., ocular X6, table, 110, 45°, would show that the sketch 
was made at the level of the table, with the mirror bar at 110, and 
the camera mirror at an angle of 45.° 
