10 
MENDENHALL. 
not a fixed quantity, but that on the contrary it varies through 
a small range during a period somewhat greater than a year. 
It is believed that if this hitherto unsuspected variation be 
applied to the parallax observations referred to above, the 
seeming absurdity of the result will vanish. 
If astronomy, the foremost of the exact sciences, is not 
free from the fault of basing conclusions upon insufficient 
premises, it will not be expected that among other sciences 
the evil will be of less magnitude. 
When we consider the sciences of heat, light, electricity, 
magnetism, and other specially investigated properties of 
matter, all of which are usually included under the general 
head of “ Physics,” we meet with a formidable rival of 
astronomy in the extent to which they are entitled to be 
considered as exact sciences. 
Physics treats of all the properties of matter, not omitting 
that wdiich is the special domain of astronomy. As if this 
were not enough, the demands upon the science are such that 
it must also deal with that which is not matter or, at least, 
is not matter in the ordinarily accepted sense. Although 
physics deals with all of the properties of matter, no physicist 
knows them or, possibly, half of them. Perhaps not one of 
them is entirely and completely known. It w T ould seem, 
therefore, that this science must of necessity be one of uncer¬ 
tain conclusions. That it is far from deserving so sweeping 
a criticism is due to the fact that the properties of matter are 
not so closely interrelated as to make it impossible to isolate 
one or more of them in experiment, and thus the problem is 
vastly simplified. It is probably impossible to do this rig¬ 
orously in any case, so that there must always remain a 
small residuum of uncertainty due to the interference of un¬ 
known or imperfectly understood properties of matter. 
Thus it is possible to treat a mass of matter as though 
it possessed mass only, ignoring its electrical, magnetic, or 
optical properties, its relation to heat, its elasticity, and other 
physical characteristics, and investigate its behavior under 
the law of gravitation alone; its optical properties may be 
