1886.] NEW YORK ACADEMY OF SCIENCES. 261 



etta, at Depauw, at Nashville, and at St. Louis, also at Franklin 

 and Marshall College, and at Doane College, in Nebraska; at Co- 

 lumbia College, Ann Arbor and Madison, Wis., and at one or two 

 other institutions vvhicli escape me for the moment. Several others 

 are also at this moment in process of erection. Every one of 

 them has a telescope from six to thirteen inches aperture, with 

 accessory apparatus sufficient, in the hands of an astronomer, 

 for useful scientific work. 



Instruments. 



A large number of new instruments of great power have been 

 constructed. We mention the great thirty-inch refractor of 

 Pulkowa, the twenty-six-inch of Charlottesville, and the twenty- 

 three-inch at Princeton, for all which the lenses were made by 

 our own Clark. We add the great Vienna twenty-seven-inch 

 telescope by Grubb, and the twenty-nine-inch object glass by the 

 Henrys, made for the Nice observatory but not yet mounted; 

 also the nineteen-inch telescope at Strassburg by Merz. Grubb 

 has also at present a twenty-eight-inch object glass under way 

 for the Greenwich observatory, and Clark has nearly completed 

 the monstrous thirty-six-inch lens for the Lick observatory. 

 There never was a decade before when such an advance in optical 

 power has been made. 



Great 7'eflectors have been scarce, the only ones of much im- 

 portance constructed during the time being the twenty-inch 

 instrument at Algiers, and Mr. Common's exquisite three-foot 

 ;, which he has lately sold to Mr. Crossley in order to 



make way for one of five-feet diameter now, I believe, under 

 construction. The old three-feet and six-feet instruments of 

 Lord Kosse have been improved in various ways, and are 

 still in use — especially in work upon lunar heat. Among 

 newly invented instruments we mention the meridian 

 photometer of Pickering, the wedge photometer of Pritchard, 

 the almucantar of Chandler, the concave diffraction grating 

 of Rowland, and the bolometer of Langley — all but one 

 American. Repsold's improvements in the micrometer, in the 

 heliometer, and in the mounting of equatorials should also be 

 mentioned here. 



As to new astronomical methods, enough has been already said 

 about photometry and astronomical photography. It is plain 

 that we are entering upon a new era. 



Literature. 



Astronomical literature has flourished. Among the books of 

 the last ten years, important in one way or another, I mention 

 in the first rank the great work of Oppolzer upon orbit calcula- 



