HARDWICKE'S SCIENCE-GOSSIP. 



18- 



the earnest gaze of dying man leaves its impression 

 on the retina in such wise, that an expert may find 

 the portrait of a murderer thus photographed on the 

 eye of his victim. Although this is too ridiculous 

 for sober discussion, something actually has been done 

 which at first sight may seem to resemble it. 



The "British Journal of Photography" tells us 

 that Dr. Rosebrugh read a paper at the Canadian 

 Institute, in which he described the following feat. 

 A glass transparency was illuminated by the sun, 

 and reflected into the eye by a piece of plate glass 

 placed at an angle of 45 to the rays passing through 

 the transparency. The image thrown on the retina 

 was viewed through the glass from a point situated in 

 the axis of the eye, and a small camera and lens there 

 placed gave an image of the picture on the retina, and 

 it is stated that photographs taken with about five 

 seconds exposure by this means were exhibited to the 

 meeting. " Nature " further states two series of pho- 

 tographs were taken ; that the first simply presented 

 views of the optic nerve and retinal blood-vessels ; 

 the second series showed not only the retina of the 

 eye, but also an inverted picture of objects to which 

 the eye was directed, pictured upon the retina. 



The resemblance of this to the pretended exploits 

 of the story-tellers is very remote, as it does not 

 include the absurdity of the latter, which consists in 

 assuming that the retinal picture is a something 

 which remains fixed. Dr. Rosebrugh simply takes 

 a copy of a picture actually existing and while it 

 exists, an existence which terminates immediately the 

 light rays and the picture from which they proceed 

 are withdrawn. 



The Sense of Smell in Leeches. — Mr. A. G. 

 Bourne, writing from Ootacamund, Nilgiris, to 

 "Nature," describes a curious experiment that may 

 be repeated by some of the readers of Science- 

 Gossip. He picked up a stone from the muddy 

 bed of a shallow torpid stream and a few minutes 

 afterwards observed a number of leeches swimming 

 near the spot. On the next day he stirred the 

 surface of the mud with a stick ; no leeches ap- 

 peared. Afterwards he washed his hands in the 

 water without disturbing the mud, and presently a 

 number of leeches rose from the mud in which they 

 live about a foot deep, and swarmed about. The 

 question asked by Mr. Bourne is, whether the leeches 

 were guided by smell or taste of the hands. As 

 evidence of the powers of scent of land leeches, he 

 refers to their coming from the banks on either side 

 of a wide road to a man or horse. The painful ex- 

 perience of all who have travelled far in Ceylon 

 confirms Mr. Bourne's conclusions so far as land 

 leeches are concerned, and it will not be difficult to 

 make further experiments upon our pond leeches in 

 England, as they may be so easily domesticated in a 

 small aquarium. 



An Unimprovable Instrument. — In "Ciel et 

 Terre " of July 1st is a history of the Barometer, from 

 which I translate the following: " This instrument 

 found itself at once produced at the very first step in 

 its most perfect form. Much time, much thought 

 and work have been expended upon the endeavour 

 to improve, and even |to supersede, the old tube of 

 Torricelli, but in spite of all the fertility of invention 

 thus employed, it remains the simplest and the best 

 of all the barometers we now possess." 



It was a fellow-countryman of Torricelli's that 

 exclaimed with profound truth, " Questo semplice 

 quanto e difficile.'''' Oh, how difficult is simplicity ; 

 and he might have added, " e quanto e eccellentc." 

 The excellence of Torricelli's tube depends on its 

 simplicity ; and the same applies to almost every 

 other contrivance, and without any exception to 

 every exposition of natural truth. 



In all our meteorological observatories the simple 

 straight tube, filled with mercury and inverted over a 

 small cup or cistern of mercury, is used. The vernier 

 is applied to reading the scale. Methods of filling the 

 tube for the more perfect exclusion of air, and for 

 adjusting the level of the mercury in the cistern have 

 been applied, but still the simple tube remains as at 

 first. 



The syphon barometer, with pulley and wheel and 

 index on face, the diagonal barometer, the water 

 barometer, glycerine barometer, are interesting, and 

 supply longer scales, but lack the reliability of the 

 simple tube charged with mercury. Even the 

 aneroid, though so elegant and portable, can only be 

 relied upon when periodically corrected by com- 

 parison with Torricelli's tube. 



HOMCEOPATHIC TREATMENT OF LUNATICS. — 

 " The Scientific American " relates an interesting 

 cure of insanity on the principle of similia similibus 

 curantur. Two monomaniacs, each with troublesome 

 but very different delusions, were set to watch each 

 other, each being made acquainted with the other's 

 mania, and instructed to prevent consequent folly 

 and mischief. Each accepted the charge and its 

 responsibilities, each pitied the other poor fellow, 

 watched him closely, and while doing so each forgot 

 his own delusion and both were finally cured. 



I remember a somewhat similar case in an asylum 

 to the inmates of which I occasionally lectured. His 

 delusion was that he had inherited a very large 

 freehold estate, including the whole of Europe. He 

 was confined in consequence of his attempts to enforce 

 his rights by eviction of occupiers, but was much 

 mollified after a while by being indulged in a lesser 

 or secondary delusion of supposing himself to be the 

 proprietor and director of the asylum. 



The subject of my lecture on one occasion was 

 " An Ascent of Mont Blanc." After the lecture he 

 came to me and thanked me most courteously for my 

 kindness in amusing and instructing his unfortunate 



