30 Dr. Wentwortu Erck on the Satellites of Mars. 
favourable for observation during the present century. But even at this most 
favourable opposition, the period of time during which the planet was sufficiently 
near to us to admit of his satellites being seen, did not exceed a couple of months. 
Again, it must be remembered, that it is only at, or near to, the time of greatest 
elongation from the primary that the satellites can be seen—even with the 
Washington telescope they were only visible when within 30° of greatest elongation ; 
or, in other words, only during one third part of their orbits. 
2. Regarding the other condition of visibility, the possession of telescopes of 
exquisite defining power, it must be borne in mind that the earliest telescopes at all 
capable of dealing with such objects as the satellites of Mars were the reflectors of 
Sir William Herschel wherewith the satellites of Uranus and the inner satellites of 
Saturn were discovered. 
Sir Wm. and Sir John Herschel have left on record several thousand observations 
of what they saw, but judging from what they failed to see, e.g., the companions of 
Sirius and Antares, and the Sixth Star in the trapezium, we may pretty safely say 
that no telescope of theirs would have shown the satellites of Mars. 
Next, having regard to reflecting telescopes, we come to those of Lord Rosse, 
which were constructed chiefly with a view to the examination of the Nebule, and 
in these telescopes perfection of definition seems to have been in some degree 
sacrificed to the collection of an immense quantity of light—such telescopes are not 
adapted to show the satellites of Mars. Then comes the two feet reflector, con- 
structed by Mr. Lassell in 1846, with which were discovered the Hyperion satellite 
of Saturn, the satellite of Neptune, and the two innermost satellites of Uranus; 
and this, I believe, to have been the first reflecting telescope capable of showing the 
satellites of Mars. 
Another telescope of four feet in diameter, constructed by Mr. Lassell in 1860, is 
no longer in existence ; and the four feet reflector at Melbourne, was unfortunately 
disabled at the time of the late opposition. 
Lastly, we come to the modern silver-on-glass reflectors, introduced by Foucault ; 
several of these are undoubtedly capable of showing the satellites of Mars, indeed 
the only European measures of the outer satellite that I know of, besides my own, 
were made with an eighteen-inch silver-on-glass, by Calver, of Chelmsford, the focal 
length of which is only five times the diameter. 
Turning our attention next to refracting telescopes, we find that the first re- 
fractors at all capable of dealing with such objects as the Satellites of Mars, were 
those of Fraunhofer, circa 1827. Of these the most famous was that at Dorpat, 
9.5 inches in diameter, employed by the elder Struve in the construction of his great 
catalogue of double stars. This telescope has, probably, done more recorded work 
than any other. 
Next in point of date come three great object-glasses, of about 12 inches in 
diameter, by Cauchoix of Paris, inter 1827 and 1830. These excited great attention 
