18 79.] : Microscopy. 59 
sight suppose. They are entirely uninjured, and their purposes for 
microscopic study remain as good as by any other process. Ten- 
der specimens in every case must be tenderly treated. This 
mode of ae has been followed by several microscopic 
friends in my vicinity for two or three years, and all the specimens 
so treated have been remarked for their beauty and excellence.— 
F. C. Clark, Providence R. T. 
LIMITS oF ACCURACY IN MEASUREMENTS WITH THE MICROSCOPE. 
—Before we can safely draw pti from a given series of 
measurements, it is necessary to know within what limits their 
errors can be determined. A dank and direct way to do this is 
to compare the measurements of the same space made by differ- 
ent observers and under entirely different conditions. I may get 
results which show an agreement, zzZer se, quite within the limits 
of the accuracy required, and which are yet wide of the truth. 
But if another observer obtains substantially the same results 
from a series of measurements made under entirely different con- 
ditions, the inference of their general correctness may be drawn 
with tolerable safety. 
One must draw a sharp distinction between absolute accuracy 
and an appearance of accuracy. For example, the head of the 
screw of my dividing engine can be set to correspond to a motion 
of one billionth of an inch with entire certainty as far as the 
mechanical indications of this degree of accuracy are concerned, 
and yet previous to May, 1877, the actual errors of a given ruled 
plate amounted under certain conditions to yoy of an inch. Even 
now, after four epochs o of improvement, 1l can hardly say of a 
given space that it is certainly true within yyy of an inch, until 
I have made a special investigation of it with my comparator. 
In carrying forward this investigation I was fortunate in securing 
the coöperation of Prof. Edward W. Morley, of Hudson, Ohio, 
an observer who possesses in a high degree the three requisites, 
patience, care and skill. I.ruled five plates of bands, plates No. 
and No. 2, having spaces of sy and êy of an inch, respect- : 
ively. These plates were ruled just as, I regret to say, all plates were 
ruled previous to May, 1877, without any attempt to correct the- 
errors peculiar to the screw and its mounting. For four years 
previous to this date every effort was made to correct these errors 
by mechanical adjustments. After this date I deliberately aban- 
doned all attempts to do this. Instead, I resolved to admit the 
existence of these errors, and after determining their value, I 
adopted a device for correcting them during the process of ruling. 
Plate No. 3 was ruled like No. 1, but with these systematic cor- 
rections applied. My next improvement consists in adopting a 
device for correcting not merely the systematic errors depending 
on one revolution of the screw, but also the errors peculiar to 
particular parts of the screw. Plate No. 4 consists of 1o1 lines 
- 
‘separated by an interval of yyy of an inch, and freed as sad ae 
