418 TRANSACTIONS OF THE 



this fact became known to Galileo, who used it in his discoveries. That 

 lustruraent was not a telescope — it was like our modern opera glass, to 

 which our own eye is the eye piece. Our telescope depends on a single 

 lens. Galileo saw the moons of Jupiter with his opera glass, magnifying 

 only three or four times. Huygens saw one of Saturn's moons with his 

 glass. Cassini's telescope was composed of an object glass placed on top 

 of a pole, made managable in connection with a short tube below by means 

 of cords, so that he could by means of the tube at focus catch views of 

 planets and stars. He had a negative eye piece on the tube, composed of 

 two lenses. He saw four of the moons of Saturn. Herschel saw the sixth 

 moon of Saturn. Sir John Herschel, at the Cape of Good Hope, saw the 

 sixth, but not the seventh certainly — only a glimpse ! the atmosphere very 

 clear too. The moon is caught in the motion of Saturn past a star, being 

 with its planet, so that we thus discover it to be not, a star, and therefore 

 a moon of Saturn's. Lassell's Liverpool Reflector, shows Saturn's eighth 

 satellite. That telescope is more space penetrating than Plerschel's great 

 forty feet reflector. 



Lord Rosse, with his great reflector of six feet diameter, has said noth- 

 ing about the moons of the planet Herschel. Rutherford's has seen two of 

 those satellites. Mitchell's twelve inch object glass, at Cincinnati, sees 

 Saturn's three rings, two solid and one apparently nebulous — as some think 

 all matter was before it condensed into the present bodies. Some suppose 

 that the present nebulous ring of Saturn will ultimately break up into 

 satellites. 



The question is ofter repeated, why have we not larger and more power- 

 ful telescopes ? I will assign the reasons as at present understood. An 

 object glass of great size is exceedingly difficult to make of adequate purity 

 and perfectly homogeneous. One of fifteen inches, one of twenty-four at 

 Paris, but not in useful condition, and one of twenty-nine inches at the 

 Crystal Palace, England, on exhibition. Glass has been poured on an iron 

 table, then rolled out by metal roller as true as possible. Craig made ob- 

 ject glasses of this by cutting out a piece and placing it so that while 

 heated to sufficient pliability atmospheric pressure would give it proper 

 form — there were strise in it. He said that Saturn was discrete looking, 

 something like our pavement. He searched for Venus' moon in vain. The 

 Cambridge fifteen inch reflector gives fine view of Orion, &c. 



To make good object glasses we melt about 800 lbs. of glass in a cruci- 

 ble, heat it so high that it will pour like water almost, and keep stirring it 

 in order to make it homogeneous and of as uniform density as possible. 

 This stirring is difficult because the metals become too soft, and as we use 

 stirrers of pipe clay, which occasionally touchiug the sides of the crucible, 

 rub off particles which injure the glass — the effect is seen in the form of 

 spots with three tails, streaks, &c. To render this glass strong it is left 

 to cool gradually, which anneals it. When cold we break it to pieces and 

 out of the purest piece in it, we make the object glass (the flint half of it). 

 The other half is made of greenish tinted crown glass — why called a-own I 



