PROCEEDINGS OF SOCIETIES. 235 
sizes—a Fourth-metre, a Fifth-metre, and a Sixth-metre across. 
After this has been once for all done, it is wonderful with what 
precision eye-estimates of size can, after a little practice, be made: 
the eye quickly coming to be able with ease to hit off within a 
tenth of any of the standard measures. 
At present microscopists comparatively seldom resort to mea- 
sures ; and, when stated to one another, they are apt to require 
separate attention to appreciate them. If a better system were 
introduced, it is probable that, estimations being made and under- 
stood without effort, they would soon come to be made habitually, 
and so contribute very much to scientific accuracy. 
The following measures will serve as examples :— 
The colourless dises in blood are about 8 VIth-metres across in 
vertebrates, and about 10 in reptiles. 
The red discs are about 2 VIth-metres across in the musk-deer, 
the animal in which the smallest have been found ; 8 VIth-metres 
in man ; 6 by 12 VIth-metres in the crow ; 7 by 12 VIth-metres in 
the hen; 14 by 23 VIth-metres in frogs; and 77 VIth-metres, 
which is nearly 8 Vth-metres in the proteus, the reptile in which 
the largest have been observed. 
A Vth-metre, then, is a little more than the diameter of a disc 
of human blood. It is of so convenient an intermediate size in 
reference te microscopical magnitudes that it will be found a good 
plan to make a practice of registering all determinations in this 
measure, so as to have nothing but numbers to write down. 
Thus, for example :— 
: Vth-metres. 
Diameter of Volwox globator...... =80° (which may be read 8 IVth-metres.) 
Red discs of human blood ......... = 0°8 (to be read “ 8 VIth-metres.”) 
Ditto of frogs) ......2...0000 1:4. by 2°3 (to be read “14 VIth-metres by 23.”’) 
Cilia have been observed ranging 
in length from ............from 5 to 0°2 (from 5 Vth-metres to 2 VIth-metres 
—a range analagous to one from 
5 feet to about two inches.) 
The closest interval between the 
lines on mother-of-pearl is about 0:3 (3 VIth-metres.) 
The length of lacuna in human 
bone varies from...........0s....> 1to1‘8 (from 10 to 18 VIth-metres.) 
Breadth of ditto from............ 0:3 to 0°6 (from 3 to 6 VIth-metres.) 
_ The interval (according to Prof. W. Smith) between the mark- 
ings on— 
Pleurosigma formosum...... (diag.) = 0°07 (7 VIIth-metres.) 
x strigile ... ...(trans.) = 0°07—(to be read “ rather less than 7 
Vilth-metres.) 
3 Balticum...... (trans.) = 0:06+ (rather more'than 6 VIIth-metres.) 
- attenuatum...(trans.) = 0°06+(Ditto.) 
= hippocampus (trans.) = 006+ (Ditto.) 
iv strigosum ...... (diag.) = 0-06—(rather less than 6 VIIth-metres.) 
’ quadratum (Nav. an- 
gulata, Soll.) (diag,) = 0°06 —( Ditto.) 
= elongatum (Nav. lini- 
ata, Soll.) ...(diag.) = 0-05+(rather more than 5 VIIth-metres.) 
7 lacustra ...... (trans.) = 005+ (Ditto.) 
