i87i-: 



Light. 



417 



centred in and applied by a third meniscus lens of flint-glass, which is added 

 to two other lenses when the telescope is to be used for photographic work, 

 and which is taken away when the instrument is to be employed in the more 

 usual work of visual observation. This arrangement is found to be so conve- 

 nient, and to act so admirably, that there is no doubt it is the form which will 

 henceforth be adopted in all important observatories, to enable their large 

 equatorial instruments to be turned to account in occasional photographic 

 work. The third telescope which has been photographically corrected is of 

 the same form of construction, and has an effective aperture of 6| inches. This 

 instrument was made for the United States Government Eclipse Expedition, 

 and was used by the expedition in Catania in December last. 



Microscopy. — Messrs. Powell and Lealand have recently constructed a 

 £th objective, in which, in addition to the usual arrangement for correction of 

 the aberration caused by the glass cover by separating the front lens, another 

 collar is added which acts in a similar manner upon the posterior combination; 

 the result is that the adjustment of the objective can be far more correctly 

 made than by the old plan, and a greater thickness of cover glass can 

 be used. The above object-glass worked well through a cover of coi inch 

 in thickness. 



The erecting binocular microscope of Mr. J. W. Stephenson, although 

 originally contrived for the purpose of dissection under low powers, is capable 

 of performing well with objectives up to £th, the definition with this power 

 being but slightly impaired. For this purpose it is found convenient to omit 

 the upper prism, which was added chiefly for the purpose of permitting 

 the stage to remain in a horizontal position, while the bodies were con- 

 veniently inclined for observation. A modification of this upper prism also 

 permits the employment of two pairs of bodies, which enables observations to 

 be conducted by two persons simultaneously : the use of this latter form only 

 involves the loss of a small portion of light, which is easily compensated for by 

 increased intensity of illumination. This is decidedly the best binocular 

 introduced since Mr. Wenham's. 



Two binocular microscopes acting upon the principle of dividing the pencil 

 by double refraction by means of a prism of calc-spar have been recently con- 

 trived. In one, the invention of Mr. F. A. P. Barnard, President of Columbia 

 College, New York, a prism of calc-spar (Fig. ii),abcd, cut to the angles 

 indicated in the diagram, is divided 



at be; the surfaces are separated by Fig. ii. 



a small interval : in practice they are 

 kept apart by a single thickness of 

 tinfoil introduced at each of the angles. 

 The prism, F g h, is of flint glass, 

 with a refracting index of 1*56 or 

 higher ; it is placed with the side F H 

 parallel to B c. A ray incident per- 

 pendicularly upon d c and doubly 

 refracted by the prism is resolved into 

 two rays, E and o, of which the first is 

 transmitted and the second reflected 

 by b e, passes perpendicularly through 

 the two surfaces b c and f h, and is a second time reflected by g h, and 

 finally emerges at right angles from the face, f g. The author states, that 

 with a Wales's objective marked i-30th but more exactly rated i-25th, with 

 the b eye-pieces, the "Providence" Grammatophora is resolved with great 

 facility. 



The binocular adaptation of Mr. C. D. Ahrens consists of a double image- 

 prism of calc-spar of peculiar construction, a a, with a piece of thin parallel 

 glass cemented on each end, which not only preserves the surfaces from 

 injury, but also improves the definition ; the ray e from the object-glass 

 is divided in each portion of the prism into two rays, but only the extraor- 



VOL. VIII. (O.S.)— VOL. I. (N.S.) 3 H 



e 







B 



30' 



y^ 1 







90° 



ioi'l mi 





H 



