November 1, 1889.] 



KNOWLEDGE 



of colour blindness found 617 colour-blind people, or 4-156 

 per cent., out of the 14,846 examined by them. Dr. 

 Bickertou, among 3,087 men tested by him, discovered 

 that 105 or 3-40 per cent, were suffering from this visual 

 affliction. 



Tlie method of examination in all cases was that recom- 

 mended by Professor Holmgren. It consists in each 

 individual picking out from a large number of differently 

 coloured wools all those skeins which have in them any 

 tint of the particular test colour placed before him. The 

 large majority of people would pick out to the very last 

 every skein wliich had any tint of green ui it, no matter 

 how delicate ; but a certain number would choose some 

 colour, such as grey, yellow, or red, to match with the 

 green, or they would omit to pick out the greens. In this 

 manner their weakness of colour perception would betray 

 itself to the examiners, who would then proceed to apply, 

 under the same system, still more crucial tests. That 

 colour-blind tests should be absolutely fair to the tested 

 one, it is of the highest importance to distinctly observe 

 the difference between the manner in wliich the colour- 

 blind .set's and the manner in which he nanifx colours. The 

 sensation is based upon the nature of the sense of colour in 

 the organisation of the optic nerve from bu'th. The name, 

 on the contrary, is learned. It is purely conventional, and 

 depends upon exercise and habit. The names of colours 

 are naturally the objective expression of subjective sensa- 

 tions. They are regulated by the system of the normal 

 sight, and cannot, consequently, agree with that of the 

 colour-blind. They can nevertheless be learned by the 

 latter, and even applied correctly in many cases. It is no 

 doubt under this veil that many colour-blind subjects un- 

 consciously conceal their infirmity from themselves and 

 their most intimate fi-iends. Hardly any limit can be set 

 to their capacity for learning other attributes of coloured 

 objects, and of recognising and rememljering their colour 

 names through these alone. Thus they dfceiw tlieniselves 

 tliat they have gained by pHKiiir a colour perception they 

 were not born with. Persons afflicted with colour blindness 

 have been known to succeed as landscape and portrait 

 painters ; but then their colours must either be marked or 

 chosen for them, or detection of their failing may ensue. 

 A student of tlie Royal Academy who was selected, not 

 only by the authorities, but also by his fellow-students, as 

 having the best perception for form and ]rower of light and 

 shade, turned to the use of colours. In this branch of liis 

 art it was naturally supposed that lie would also exhibit 

 talent. He was allowed to take a portrait by Titian from 

 the National Gallery, and have it in a small room by him- 

 self. There he copied it to the best of his ability, and, as 

 he stated to the Principal and his brother students, no one 

 visited hira while he was at his work. His result was the 

 most perfect copy, as far as light and shade went — but all 

 shades of green had been used. 



The consensus of medical opinion goes to prove that 

 colour blindness is congenital and hereditary. Children 

 born with it do not grow out of it as they rise in years, 

 nor can the defect be palliated by any practice with colours. 

 It has been shown that this visual defect is subject to the 

 general laws that govern the transmission of hereditary 

 disease from one generation to another. In the ordinary 

 walks of life a man may go through life colour blind 

 merely at the cost of inconvenience and discomfort to 

 himself. But on the ocean and the railway, where safe 

 locomotion is always dependent upon the ability of the 

 look-out to discriminate innuediately and correctly between 

 red and green signal lights, every degree of chromatic 

 defect is dangerous alike to life and property. But of this 

 aspect of the colour-blind <iuestiou and its close relation to 



the Mercantile Marine, I propose, if the Editor \vill permit, 

 to write on a subsequent occasion. 



Persons cannot be tested for eolom* blindness too early 

 in life. The tests now employed are of such a simple 

 character that they might with very great anvantage be 

 introduced into our public schools ; and many a lad would 

 be spared the mortification of discovering in later life that 

 he has embraced a calling continuance in which is fraught 

 with terrible risk and danger to the lives and property 

 entrusted to liis care. 



ON LARGE TELESCOPES. 



By a. C. Eaxyabd. 



IT is now more than two centuries and three quarters 

 since the imagination of man was first stiiTed by the 

 invention of an instrument which enabled him to 

 magnify the face of the heavens. He had &'om a 

 very early date — fi'om Assp'ian times at least — 

 possessed lenses which enabled him to magnify the minute 

 thuigs about him to a small extent, and to see somewhat 

 more of their beauty than he could detect with the eye 

 alone. But the invention of the telescope in the first 

 decade of the seventeenth century, or, more accurately, 

 the use to which Galileo put the new-found treasure, 

 revolutionized philosophy. It was a poor little instrument 

 which Galileo possessed, gi%'ing verj- little Ught and a 

 great deal of colour. Though his best telescopes magni- 

 fied thirty-two or thirty -three times, the sharpness of their 

 images must not be compared with a modei'n achromatic 

 of simOar power. Since the days of Galileo, it has been 

 the constant ambition of many minds to de\-ise means 

 which would enable us to peer tiu'ther into space. The 

 progress was at first very slow, longer and longer tele- 

 scopes were used, till instruments of over 200 feet focal 

 length were employed ; they had, of comse, no tube. 

 The object glass was pulled up and down the side of a 

 mast, and the observer stood on the gi'oimd with the eye- 

 piece held to his eye, and a string in his hand to turn the 

 object glass towards him. Even in still weather such 

 telescopes must have been very difficult to use with the 

 constant varying height of the object above the horizon, 

 and the shiftuig of the object glass and observer necessary 

 to keep the object in focus. But with such an instrument 

 (of 212j feet focal length) Bradley measured the diameter of 

 \'enus m 1722 ; and Casini observed the great dinsion 

 in Saturn's ring, and four of his satellites. Titan, the 

 largest of the satellites, had been discovered in 1655, with 

 a 12-foot tube, by Huygheus. 



The first material step in advance was due to the 

 suggestion of a Scotchman, -lames Gregory, who. in his 

 Oiititit I'rviiuitit, published in 1603, proposed that the 

 rays of light from a remote object should be received 

 by a concave parabolic speculum and be reflected back 

 by a smaller elliptic speculum through a hole in the centre 

 of the larger reflector ; but he failed m his attempt to 

 obtain a sufficiently true parabolic figure. And the first 

 person who actually succeeded in making a reflecting tele- 

 scope was Sir Isaac Newton. Although this invention got 

 rid of the chromatic difficulty (for rays of all colours are 

 brought to the same focus by a reflector), very little 

 astronomical work was accomplished with the new instru- 

 ment of research until after the invention of the achromatic 

 telescope, the idea of which had been conceived by 

 Newton. But owing to a curious mistake in im experi- 

 ment the cause of which is involved in mystery, he not 

 only did not invent the achromatic telescope, but he went 

 so far as to predict that such a telescope never could be 



