104 



NA TURE 



[May 31, 1906 



attention for its preservation. This is, no doubt, due to 

 the small proportion of magnesium chloride. 



\ preliminary analysis affords the following result : — 



Moisture o-i2 | MgCl., + 6H„0 0-22 



Insoluble matter .. o 65 I Na.,S6j + loHoO ... i'95 



MnCU + 4H20 38-97 j Fe.r03 " traces 



KCl 5771 " 



NaCl 0-32 I Total 99 94 



From this analysis it appears that we have to do with 

 a double chloride of manganese and potassium for which 

 I propose the name of chloniiaiigaiwkaliie, at any rate 

 provisionally until more complete studies can be made of 

 this undescribed species and proper proportions attributed 

 to the combined molecules. 



The associated minerals have been deposited as sub- 

 limates in the interspaces of scoriaceous masses forming 

 the upper part of the great cone. 



The halite probably is rich in potash, but I have not 

 yet had time to make an analysis. 



H. J. JOHNSTON-L.4VIS. 



THE PHOTOGRAPHY OF THE FUNDUS 

 OCULI. 

 T T is not surprising that the idea of photographing 

 ■'■ the fundus of the eye followed quickly upon the 

 discovery of the ophthalmoscope by von Helmholtz in 

 1851. The many attempts made by Noyes (1862), 

 Rosebrugh, Dor, Howe, Bagneris, and others on 

 animals met with only partial success, whilst Gerloff, 

 Thorner and others, who attacked the more difficult 

 problem of the human eye, obtained very inferior 

 results. The best photographs of the human fundus 

 have been taken by Prof. Dimmer, of Graz, who 

 records his experiments in the Sitzungsherichte der 

 kaiserlichen Akadeniie der Wisseiischafteii zii Wien, 



1 of apparatu 



Heft 



MatJi.-Naturwissensch. Klasse (Bd. cxiv. 

 1905). 



The chief difficulties which have to be c'ontended with 

 are (i) that of keeping the eye immobile; (2) the 

 necessity for very powerful illumination, which may 

 have a deleterious effect upon the eye; (3) the reflexes 



NO. 1909, VOL. 74] 



formed by the surfaces of the dioptric media — corneal, 

 lenticular, and fundal. Of these the complete aboli- 

 tion of reflexes has as yet proved insuperable. Prof. 

 Dirnmer has succeeded in reducing the corneal and 

 lenticular reflexes to a minimum, but the shimmering 

 reflexes at the fundus, which vary so much in difterent 

 eyes, have re- 

 sisted all efforts. 

 Nevertheless, an 

 encouraging de- 

 gree of success 

 has been at- 

 tained. 



The method 

 adopted was as 

 follows : — T h I' 

 source of light 

 was a 20-30-am- 

 pere arc lamp (l). 

 The light was 

 concentrated on :i 

 small diaphragm 

 (dj), which may 

 be regarded as 

 the immediate ^""^- 2— Tte normal fundus, 



source of illumin- 

 ation. A second condenser system (b) formed an 

 image of the diaphragm at the level of the pupil of 

 the eye (d„). The light and condenser systems were 

 centred on an axis at right angles to the optic axis, 

 the cone of rays being reflected into one half of the 

 pupil by an oblique mirror (s) immediately in front of 

 the eye. This arrangement has the advantages of 

 illuminating a maximum field of the fundus, of reduc- 

 ing the corneal and lenticular reflexes to a minimum, 

 and of leaving the other half of the pupil free for the 

 emergent rays. The last device is that adopted by 

 Bagneris, and by Wolff in his electric-light ophthalmo- 

 scope. 



The image of the fundus is that of the ordinary 

 indirect method of ophthalmoscopic examination. A 

 convex lens (c) forms a real inverted image at or near 

 its principal focus (f), according to the refraction of 

 the eye. A second convex lens (d) is used to form an 

 erect image upon the sensitive plate (pp). An oblique 

 mirror (i:i:) in the camera reflects the image on to 

 a ground glass screen (PiP,) at right angles to the 

 position of the plate until the moment of exposure ; 

 this facilitates focusing, &c. 



Fixation is maintained by an object (m) in the axis 

 of illumination, the diaphragm being covered by a 

 smoked glass. At the moment of exposure, which is 

 instantaneous, a blank diaphragm (d,) is shot into 

 position by a simple electromagnetic arrangement. 

 Orthochromatic plates are, of course, used to minimise 

 the inopportune colour of the image. 



Prof. Dimmer has photographed several normal 

 and pathological eyes. No evil effects have apparently 

 ensued, owing to the shortness of the exposure 

 required. The results are better than any previously 

 obtained, but they are far from perfect. Those who 

 saw the original photographs at the International 

 Ophthalmological Congress at Lucerne in 1904 will, 

 however, condole with the author for the inferior half- 

 tone reproductions of the Imperial .Academy. 



We are forced to admire Prof. Dimmer's pertinacity 

 in this research, at which he has been occupied since 

 iSqq. Considering the well-known dangers of very 

 bright illumination upon the retina, the mediocre 

 results obtained, and the doubtful utility in ophthalmic 

 surgery, we cannot but hope that future experiments 

 will be confined for the present to animals. 



J. Herbert Parsons. 



