234 



NA TURE 



[July 4, 1S95 



the purpose, that my suspicions of spots were entirely illusorj-, 

 and that such markings as objective features were invisible to my 

 eve with the means employed. On the worst nights I could 

 easily imagine a mottled aspect of the belts ; but with good de- 

 finition and a steady image, the tone of the belts and bright 

 equator appeared [Terfectly even and free from noticeable irr^- 

 larities. In a case of this kind the observer has to be severe with 

 himself. There is a distinct line of demarcation between what is 

 absolutely seen and what is jxissibly seen or suspected. An 

 object may be only glimiised, and yet it is certainly seen, for its 

 impressions reach the eye now and then in a form not to t>e mis- 

 taken. But with some objects the experience is different. We 

 fincy they are there, but cannot fix them w ith certainly ; ap- 

 parently they flit about like an ignis fatuiis, and are intractable 

 to our utmost efforts. Obviously in such a case the observer 

 has but one alternative, and that is to regard the objects as 

 imaginar)'. 



On Mars, as well as Saturn, small instruments have done 

 ■wonders. It is well known that the canals and their duplication 

 were discovered by Schiaparelli with a refractor of only bj inches 

 aperture. In 1S92, dunng a favourable presentation of Mars, 

 the large American telescopes showed very little either of the 

 canals or of their duplication. During the opixisition of 1894 

 the planet was better placed as regards altitude (but not so near 

 10 the earth as in 1892), and the results of observations have been 

 more satisfactory. Mr. Williams with a 6J-inch reflector, and 

 Mr. Brenner with a 7-inch refractor, have recovered many of 

 the double canals of Schiaparelli. Mr. V. Lowell, with the 18- 

 inch refractor at the observatory at Arizona, has also observed 

 many remarkable and intricate details of the planet's tojwgraphy. 

 This observer remarks that in regard to the visible markings on 

 the inner planets of the solar system up to and including Mars, 

 size of instrument is quite secondary ti> quality of atmosphere. 

 He draws the "oases" on Mars, and a large number of inter- 

 lacing lines on the planet, in Popular Astronomy for .\iiril 1895, 

 and the pictures are very effective. There are many of us who 

 would like to obtain a view of .Mars similar to what he has 

 <lepicted. Mr. Lowell notes that with the 18-inch a power of 

 420 was as high as the atmos|)here permitted to be used with 

 advantage, though drawings w ere generally made with 370. On 

 the 6-inch refractor 270 showed well, the dark and light mark- 

 ings lieing more contrasted than in the larger instrument. As 

 affecting the comparative utility of large and small telescopes, 

 Mr. Lowell remarks: " .\ large instniment is a.ssumed to be 

 necessarily suiierior to a small one, quite irrespective of what it 

 is that is to be observed. Now the fact is that there are two 

 <]uite different classes of celestial phenomena — those dependent 

 on quantity of light, and those dependent on qu.ility of definition 

 for their visibility, and the two means to these ends go anything 

 but hand in hand, hor the one, the illumination, the size of the 

 in.strument is the prime requisite ; for the other, the definition, 

 the atmosphere is the first essential. As an object-le.sson in 

 this, it is worth noticing that the biggest instruments have not 

 always given the best views of Mars. In matters of Martian 

 detail it is amply evident from the results that observer, atmo- 

 sphere, instrument, is the order of weight to be given as the factors 

 of an observation." 



I have referred to this subject without any desire to lake up 

 the cudgels on liehalf of any cla.ss of instniment, but it is .sugges- 

 tive that the large ones will not bear |X)wers commensurate with 

 their size on planetary details. Thus with the 36-inch at Mount 

 Hamilton a ixiwer of 350 has l>cen found the most effective on 

 Mars ; a similar pfiwer can \k used with advantage on glasses of 

 only 8 or 10 inches diameter. It is difficult to understanil, 

 therefore, where the su|>eric)rily of large instniments comes in, as 

 the object is sufficiently bright in small telescopes, and the latter 

 l>eing more easily manipulated and less affected by atmospheric 

 tremors, they obviously iM)ssess some distinct advantages. But 

 this interesting and im|H>rtant tpiestion is scarcely to l)e .settled 

 by a mere discussion of this sort. It is only to Ik: settled by 

 careful trials of large and small instruments, side by side, u|)on 

 the planets .Mars, Jupiter, and Saturn. If observers having the 

 appliances at command will institute v>me further conqiarisons of 

 the kind suggested, the problem might be virtually solved in a 

 short lime. Relying u|)on evidence of fragmentary character is 

 scarcely fair, since differences of eyesight anfl atmosphere come 

 into play most prominently. The most valuable eviflence would 

 Xk Inat of an oliserver who used a number of telesro|ies of 

 <llfrercnl apertures at one and (he same station. Up to the 

 •jrcscnt time it must lie confes.v:d that small instruments have 



somewhat the best of the ai^iment ; but if the unanimous testi- 

 mony of our most trustworthy oKservers a.sscrted the superiority of 

 large telescopes on bright planets, it is hard to see how they could 

 be disproved, as they alone have the effective means of judging 

 the question on its merits. W. 1". Dennim;. 



SUBJECTIVE VISUAL SENSATIONS} 



'T'HK activity of the cerebral centres which is independent of 

 ■'■ their common exciting causes, and which is termed 

 " discharge," presents indications of the character and loss of 

 their function which can be obtained from no other source. 

 I'orcmost in interest and also in importance are the sensations of 

 sight which occur without stimulation of the retina. Of these 

 the most important are two. ( i ) Those which occur at the onset 

 of epileptic fits, from the "discharge" in the brain influencing 

 consciousness, through the visual centre, before loss takes i>lace. 

 (2) Those which occur as the precursory symjitonis of the 

 paroxysmal headaches which, from their one-sided distrilmtion, 

 have been called " hemicrania," "megraine" or " iiiegrim," 

 from the freijuent vomiting, " sick headaches." and, from the 

 inhibitory loss of sight, " blind headaches." These two classes 

 fonii the subject of the lecture. 



In what part of the brain does the process occur ? The 

 impulses from the retina reach the cortex of the brain first in 

 the extremity of the occipital lobe, where, as Munk first showed, 

 the half-fields are represented in strictly local definiteness. The 

 left occipital lobe receives the impulses from the left half of each 

 retina, produced by the rays of light from the right half of each 

 field of vision. So, conversely, with the right occipital lobe. 

 To each side, impulses proceed from a very minute area around 

 the centra! point of the retina, the fixation point of the field. 

 But we cannot conceive that the functional disturbance occurs in 

 these centres, for the strict medial division in two halves is 

 absolutely ignored by the subjective sensations. Moreover, the 

 strange but certain facts of hysterical hemiana.sthesia, in which 

 there is inhibition of all the sensory centres of one hemisphere, 

 present us with remarkable evidence of the higher visual 

 function in each hemisphere. This is supported by some cases 

 of organic disease, which cause an affection of sight similar to 

 that of hysteria, and by more common cases of hemianopia from 

 disease of the hemisphere, in which there is a precisely similar 

 contraction of the remaining half-fields. The significance of all 

 these is that the early conclusions of Kerrier are correct, and 

 that, in addition to the lower, occijiital half-vision centre, there is 

 a higher centre in each hemisphere, situated in the region of the 

 angular convolution. This theory of the dmilile visual centres, 

 consisting of a combination of the conclusions of Kerrier and 

 Munk, was first slated by the lecturer in 1SS5, and has been con- 

 firmed by all the facts he has since met with. It is inilispens.able 

 for the comprehension of morbid functional .action, and, indeed, 

 for that of normal vision, but is not yet recognised by ph)siolo- 

 gists, even as hypothetical. 



The character of the function of this centre, so far as it 

 can be discerned from the facts of its loss, are of great imjwrt- 

 ance for the study of visual sensations. The two higher centres 

 seem to be blended into one in function in a manner that is 

 unique so far as our knowledge extends. If the centre on one 

 side is functionless, there is loss of sight in the periphery of Uith 

 visual fields ; there is visiim in the central third of the eye on the 

 same side, and a far smaller central area on the opposite side. 

 The only conclusion is the startling inference that either higher 

 centre can .subserve central vision in both eyes, but that peri- 

 |)heral vision depends on the co-operation of the function of 

 both hemispheres. Between the central area for which either 

 centre suflices and the peripheral area for which neither is coni|)C- 

 tent but both are needed, there is an intermediate zone in which 

 vision is sub.scrved imly by the opposite hemisphere when acting 

 alime. This gradation of functional capacity enables some fads 

 of subjective sensations to be C(miprehended which cannot other- 

 wise be untlerstood. 



Moreover, the facts suggest that the function of these higher 

 centres is quite different from that of the lower ones, and from 

 that of other cerebral centres the action of which we can study. 

 In the lower half-vision centres function is localised, so that 

 destruction of part causes absolute loss of a part of the half-field, 

 blindness of the corresponding [wrt of the retina. But partial 

 ' The Ilownuin Lecture, delivered l)efore llie Opluli.-ilniological .Suciety, 

 liy Dr. W. R. Cowen, F.R.S., June m. 



NO. 1340, VOL. 52] 



