THE REFRACTING TELESCOPE. 175 



filled with some substance having, as we have already seen, the prop- 

 erties of a solid. Now, although it is easier to conceive that all space 

 is filled with some kind of substance than to conceive it to be empty, 

 in order to account for universal gravitation, it is at least unexpected 

 that this substance should turn out to be a solid, yet the polarization 

 of lio-ht shows that a solid substance it must be, notwithstanding the 

 fact that the planets rush through it without the smallest apparent 

 resistance. 



But even this anomaly is not utterly inconceivable, for many fa- 

 miliar substances have at one and the same time the properties both of 

 a solid and a liquid — for example, pitch, rosin, and tar. We would all 

 probably consider pitch as quite a brittle solid, yet it is at the same time 

 a perfect liquid, as an incident that happened to Alvan Clark will illus- 

 trate. He once opened a new barrel of pitch, using a hatchet to crack 

 off some for use in polishing lenses ; after breaking off enough for 

 his purpose, he laid the hatchet down on the pitch which nearly filled 

 the barrel, and thought no more about it until some few weeks after- 

 ward, when the hatchet could not be found, although he distinctly 

 remembered having left it lying on the opened barrel. He thought it 

 stolen until about two years afterward, when the missing hatchet was 

 discovered at the bottom of the pitch, having sunk into it, clear to the 

 bottom, leaving no hole behind, just as a stone would sink in water, 

 only of course much more slowly. 



All who have worked with pitch know that it has the property of 

 being a slowly moving liquid ; and it is evident that this particular 

 kind of substance at least is a solid to one kind of motion, such as the 

 quick blow of a hatchet, but is a liquid to another kind of motion, 

 such as the steady pressure of a hatchet slowly descending through it. 

 That is, give it plenty of time to flow, and pitch is a perfect liquid ; 

 but hurry it, and it is a very brittle solid. 



Now, this strange substance which fills all space seems to possess 

 this peculiar double property in a vastly greater degree than does 

 common pitch, for we find that to such a quick motion as a vibrating 

 molecule it acts as a most rigid solid, but to the comparatively slow 

 and steady motion of a planet it acts as an inconceivably thin liquid, 

 allowing the planet to pass through with no apparent resistance. 



This remarkable substance, which fills both intermolecular and in- 

 terstellar space, is called the universal ether. Its properties are only 

 beginning to be learned, and will not probably be well understood 

 until such phenomena as gravitation, electricity, magnetism, and the 

 peculiarities seen in the tails of comets, are satisfactorily explained. A 

 statement, however, of a few of its observed properties is a necessary 

 prelude to a complete understanding of the telescope. 



The molecules of ponderable matter are supposed to be inclosed in 

 the ether, just as a wooden ball could be incased in the center of a 

 large block of jelly. The waves of light are supposed to be originated 



