338 
PROFESSOR K. PEARSON AND OTHERS ON 
development, and our results would in this case have been more homogeneous and 
thus more satisfactory. But simply gathering them in the fields we could only make 
a very rough approximation to uniformity in age or development. We aimed at a 
condition in which the underskin had disappeared and the gills had lo.st all signs of a 
crease or notch. Dr. Lee and I collected more than 100 mushrooms in the Great 
and Little Hampdens. With the assistance of my wife we cut 10 gills out of each 
of these, distributed, as fa,r as the condition of the mushroom would allow, uniformly 
round the axis. These gills were then placed on ruled paper, the horizontal lines 
of which served to mark the maximum length of the gill. This was marked hv 
two fine needle-pricks, one at either end. Two further needle-pricks were now made 
at the points farthest removed fi-om this length line, one on the rp^per and one on 
the lower contour of the gill. Thus Iry drawing through the four needle points lines 
parallel and perpendicular to the jjaper-ruling we obtained a rectangle approxi¬ 
mately circumscribing the gill. The length and breadth of this rectangle are 
what I shall term the length and breadth of the gill. At first I endeavoured 
to take these measurements by the use of a micrometer microscope, hut this 
method was very laborious, and gave an apparent exactness wholly out of keeping 
with the somewhat rough and ready manipulation of the gill. Even when 
the gill itself was placed under the microscope the gill length could only he placed 
parallel to the spider wii’es by a somewhat vague appreciation, and the upper 
boundary of the gill was under the lens rather less definite than without it. Accord- 
ingly we resorted to the needle-pricks and the ruling by fine lines of the above- 
mentioned rectangle. I determined, after various trials with engraved glass scales, 
&c., the length and breadth of these rectangles by the use of a pair of proportional 
compasses, set to multiply ])y ten, and an ordiiiary millimetre scale. I was thus able 
to get readings to '01 of a millimetre, which agreed well on repetition. As the 
millimetre itself was afterwards adopted as the unit of groin)ing, I do not think the 
errors of manipulation will seriously affect our results. The needle-prick method 
enabled us to deal with the mushrooms quite fresh and reserve the measuring till 
another season.^ Dr. Lee found what we may term the gill-index, i.e., the ratio of 
length to breadth for the 1070 gills, Ijy aid of a Fuller’s slide rule. We had thus three 
cliaracters to deal with—length, ijreadth, and index. Of these we considered that 
the first two would undoubtedly be largely influenced l)y the stage of development of 
the-mushroom, but we hoped that the index (like, for example, the cephalic index of 
children over two years of age) would not he much influenced by the growth factor. 
On the other hand, the correlation of lengths of gills or the correlation of breadths of 
o-ills from the same mushroom would of course be influenced l)v growth as well as 
Ill all such systems of mass-measurement, the method adopted has to be chosen with due regard to 
time and eyesight. The actual measurement of the gills occupied me about ten full days, the microscopic 
method would have taken about four times the time, even if my eyes would allow of such continuous 
working and the mushrooms could have been preserved fresh. 
