TRANSACTIONS OF THE SECTIONS. 11 



affected by indistinctness due to the disordered focus. But the author had found 

 that the eye, when looking through a lens of this description under water, has 

 not much power of accommodating itself to different distances ; and with the best 

 • of those the author had as yet constructed, the limit of distinct vision appeared 

 practically restricted to a range of about eight feet. The attempt, however, was 

 only provisional : his experiments had but very recently been commenced. It must 

 be distinctly understood that men in diving-machines or helmets do not require such 

 lenses; for their eyeballs are separated from the water by the apparatus iu which 

 they are cooped up. All that is needed by such persons is to have ordinary windows 

 of stout plate glass through which they can look: out into the medium which sur- 

 rounds them. The author's contrivance refers to the wants of divers in pearl- and 

 sponge-fisheries, to sailors who have occasion to examine the bottoms of their ships, 

 to persons who have dropped something in the water which they wish to recover, 

 and to bathers generally. To those who can swim, the author promises a material 

 addition to their enjoyment, in the possession of these eye-glasses or spectacles. It 

 is no slight pleasure to live in some degree the life of a mermaid, keeping below 

 water for a minute at a time, and seeing everything in one's immediate neighbour- 

 hood as clearly as it could be seen by leaning over the gunwale of a boat on a still 

 day, when the glare from the water was perfectly shaded. There are many amphi- 

 bious animals that see as well under water as in air. Amongst these are seals, 

 otters, hippopotami, water-rats, and diving birds of man}' descriptions. The cornea 

 of the seal is flattened, but that of the other animals appears to be 88 convex as in 

 man. By what means these other animals are able to adjust their eyes to the re- 

 quirements of water and of air indifferently, is wholly unexplained. Physiologists 

 do not seem to have been aware of the vast powers of optical adjustment which 

 the habits of these animals necessitate. 



The Refraction Equivalent of Carbon. By Dr. J. H. Gladstone, F.R.S. 



The refractive index of a substance, minus unity, divided by the specific gravity, is 

 termed its specific refractive energy ; and this multiplied by the chemical equiva- 

 lent has been termed, by Landolt, its refraction equivalent. The present commu- 

 nication was intended to show that carbon, whether as the pure element, or as a 

 part of solid, liquid, or gaseous compounds, has the same refraction equivalent, viz. 

 5, or a little more. Diamond gives 5; carbonic acid, 5*08j bisulphide of carbon, 

 5*3 ; chloride of carbon, 5T5; cyanogen, about o"2 ; many hydrocarbons, 5 ; sugar, 

 about 4-8 ; while Landolt, from the refraction equivalents of compounds differing 

 by one equivalent of carbon, determined the number 5. In some highly dispersive 

 substances a higher number was arrived at by calculation. 



On a New Form of Spect ru m-Microscope . By H. C. Soebt, F.R.S. 



The superiority of this instrument, as compared with that first proposed by the 

 author, consists in the employment of a compound direct-vision prism over the 

 eye-piece. The slit is fixed in the focus of the upper lens of the eye-piece, which 

 is made achromatic, so that all parts of the spectrum may be distinctly seen at the 

 same time. By using a binocular microscope, the inclined tube can be employed 

 as a finder ; and on arranging so that a minute object is in the centre of the field, 

 it will be directly in front of the slit fixed in the eye-piece of the other tube. On 

 looking through this eye-piece it is easy to see that the object is properly placed 

 in front of the slit ; and then, on placing the prism on the eye-piece, as if it were 

 a Nicol's prism, the spectrum of the object can be seen to great advantage. This 

 compound analyzing prism consists of two right-angled prisms of flint glass, between 

 which is a rectangular prism of crown glass, and at each end a crown-glass prism 

 of about 75°, all cemented together with Canada balsam. Arrangements are also 

 made in the instrument, by means of a reflecting prism covering half the slit, so 

 that the spectrum of a minute object placed on the stage may be compared with 

 that of a larger object placed mi a stage attached to the side of the eye-piece, and 

 thus their difference or identity may be seen at a glance. It is thus easy to compare 

 tlie spectra of minute crystals and of their solutions, to study Hie Bpectra of small 

 coloured blowpipe beads, and, in fact, accurately examine the nature of the light 



