We shall be in a better position a few years hence to estimate the value of great telescopes; for the principal instruments of this class have only been completed a short time. Judging from the statements of some of the observers, who are men of the utmost probity and ability, certain of the large instruments are capable of work far in advance of anything hitherto done. Definition, they say, is excellent, notwithstanding the great increase of aperture. The old stumbling-block appears, therefore, to have been removed, and astronomy is to be congratulated on the acquirement of such vastly improved implements of research. Even should the large telescopes continue to prove disappointing in certain branches, they may certainly be expected to maintain their advantage in others. They will always be valuable as a corrective to smaller and handier instruments. For special lines of work in which very small or very faint objects are concerned, considerable light-grasping power is absolutely required; and it is chiefly in these departments that large instruments may be further expected to augment our knowledge. In photographic and spectroscopic work they also have a special value, which late researches have brought prominently to the fore. The telescopes of the future will probably surpass in dimensions those of our own day. The University of Los Angelos, in California, propose to erect a 42-inch refractor on the summit of Wilson’s Peak of the Sierra Madre mountains, which is 6000 feet high and about 25 miles from Los Angelos. In reference to this contemplated extension of size, it may be opportune to mention that large objectives do not transmit light proportionately with their increased diameter, owing to greater thickness of the lenses, which increases the absorption. The Washington objective of 25·8-inch aperture is 2·87 inches in thickness, and more than half the light which falls upon it is lost by absorption. On the other hand, specula, with every enlargement of aperture, give proportionately more light-grasping power, and their diameters might be greatly increased but for the mechanical obstacles in the way of their construction. Mr. Ranyard expresses the opinion that “with the refractor we are fast approaching the practical limit of size.” After referring to the Washington object-glass as above, he says:—“If we double the thickness, more than three quarters of the light would be absorbed and less than one quarter would be transmitted. The greatest loss of light is only for the centre of the object-glass; but in all parts the absorption is quadrupled for a lens of double aperture.” If, therefore, future years see any great development in the sizes of telescopes, it will probably be in connection with reflectors; for the loss of light by absorption in the thick lenses of large refractors must ultimately determine their limits. Mr. Calver says:—“The light of reflectors exceeding 18 inches in diameter is certainly greater than