326 PROCEEDINGS OF THE AMERICAN ACADEMy 



" In making use of single altitudes for the above times, it became 

 necessary to determine the error of coUimation of the astronomical 

 quadrant, the diameter of the wires and the distance between them, 

 which occasioned a delay that prevented the observations being sea- 

 sonably forwarded to Mr. Bowditch, to be inserted in his valuable me- 

 moir on this eclipse." 



Dr. O. W. Holmes presented the following communication 

 " On the Use of Direct Light in Microscopic Researches," and 

 exhibited at the same time a model of a new horizontal mi- 

 croscope. 



" Three points require attention in constructing a compound micro- 

 scope. First, the lenses ; secondly, the illuminating apparatus ; thirdly, 

 the mechanical arrangements for insuring stability in the requisite posi- 

 tions, and accuracy, ease, and convenience in the necessary movements. 



'■' The lenses have been brought to great perfection by the opticians 

 of Europe, especially of England. In our own country, Mr. Spencer 

 of Canastota has entered into successful competition with the most 

 eminent among them. The extraordinary merits of his lenses have 

 been manifested in various comparative trials, the results of some of 

 which have been made public. In a short visit I recently made to 

 Canastota, I carried with me the one-eighth and one-twelfth objectives 

 belonging to the instrument made by Ross for the Lowell Institute, the 

 use of which objectives had been kindly allowed me by the Curator. 

 In a careful comparison of them with a one-fourth and one-eighth 

 made by Mr. Spencer, especially on the delicate tests Navicula Spen- 

 cerii and Grammatophora, the superiority of Mr. Spencer's glasses was 

 unquestionable. 



" Next in importance to the perfection of the lenses, and even more 

 important, in Sir David Brewster's opinion, is that of the illuminating 

 apparatus. The greater number of recently attempted improvements 

 relate to this part of the instrument. Many of the new contrivances 

 are expensive, complicated, and somewhat difficult of management. 

 If the same or better results can be obtained by easier means, it would 

 be a movement in the right direction, which is always from complexity 

 towards simplicity. 



" The common mode of examining opaque objects is to receive the 

 light directly upon them as it comes from its source, or concentrate it 

 upon them by a lens. The use of a reflector for examining is less 

 frequently resorted to. 



