399 



There exists between them the most perfect harmony. They always 

 fall asleep at the same moment, and it is impossible to wake the one 

 without also waking the other. 



The author adverts, in the course of the paper, to the question 

 whether they were the produce of a single or a double ovum ; and 

 also into that of the possibility, at some future time, of effecting their 

 separation with safety to themselves ; and he concludes by bearing 

 testimony to the uniform kind treatment they have received from 

 Capt. Coffin, Mr. Hunter, and Mr. Hale, who have evinced on all 

 occasions the greatest anxiety for their welfare and happiness ; and 

 to the liberal manner in which they have always afforded access to 

 men of science for promoting any object of philosophical inquiry. 



On some Properties in Achromatic Object-glasses applicable to the 

 Improvement of the Microscope. By Joseph Jackson Lister, Esq. 

 Communicated by Dr. Roget, Secretary. Read January 21, 1830. 

 [Phil. Trans. 1830, p. 187.] 



The principles on which the reflecting, and also the achromatic 

 refracting telescope are constructed, have been recently applied with 

 considerable success to the microscope, and have added much to the 

 power of that instrument. The author speaks with much commen- 

 dation of the peculiar construction adopted in Mr. Tulley's com- 

 pound achromatic microscopes, consisting of a combination of object- 

 glasses of short focus and large aperture, the curvatures of which are 

 such as very nearly to equalize the refractions produced by each. 

 As the magnitude of the aperture, he observes, is valuable only in 

 proportion to that of the pencil of light which it admits, the latter 

 circumstance is that which chiefly claims attention ; and as it is often 

 erroneously estimated, a method is pointed out of ascertaining it with 

 sufficient exactness for every practical purpose. He then enters into 

 a detailed description of the several parts of an instrument in his 

 possession constructed on the principles he recommends, referring to 

 the drawings which accompany the paper. The magnifying power 

 may be varied at pleasure, either by drawing out the tubes contain- 

 ing the eye-pieces, or by substituting an eye-glass of different power, 

 or differently combined ; and by these changes an uninterrupted 

 range of amplification is obtained from 35 to 800 diameters. No 

 sensible difference as to distinctness is observable, whether the effect 

 is produced by changing the eye-piece, or varying the length of the 

 tubes. The construction of the instrument admits of the utmost 

 variation of magnifying power without the risk of losing sight of the 

 object viewed : and every part which relates to the illumination being 

 wholly detached from the stage, ample opportunity is afforded of 

 rapidly moving the objects, and bringing into view a succession of 

 them, while the light remains the same. Minute directions are given 

 for the employment of the instrument, and its application to various 

 purposes ; and great stress is laid on the importance of a skilful ma- 

 nagement of the light. 



