Dec. 



1884.] 



♦ KNOWLEDGE 



477 



work. The stage on which the slides or othei- object- 

 holders rest is often made to revolve. This is useful when 

 polarised light is employed, and also to exhibit the effects 

 of illuminating objects by light falling at different angles. 

 Thus the beautiful scales of Morphomenalaus exhibit a 

 fine blue tint when the light strikes in one direction, and 

 when the stage is partly revolved the colour is a brownish 

 drab. Many minerals — hypersthene, for example — show 

 brilliant colours in one position and none in another. 

 There is also a great convenience in a revolving stage, to 

 bring objects into a convenient position for studying or 

 drawing. Perhaps a rotifer is stretched out at an angle 

 that does not show it to advantage, and would appear 

 awkward in a sketch. The rotating stage enables it to be 

 seen upright 



All mi.Toscopes are supplied with a mirror below the 

 stage to throw light up, and the best have two mirrors — 

 one flat and the other concave. These should be mounted 

 so that they can be moved considerably on one side, and 

 slanted so that a very oblique light can be sent through 

 any translucent object, which then appears luminous upon 

 a dark ground. 



An achromatic condenser fitting under the stage is of 

 great use for the best display of delicate structures. It is 

 jirovided with stops which keep out more or less of the 

 central rays, and little holes to let only small pencils 

 through. The cheaper instruments are not adapted to 

 carry this apparatus ; but, if the price can be afforded, it is 

 better to have one that will, though a beginner is better 

 without it until he has learned to make the best use of 

 the substage mirror. The condenser should give a good dark 

 ground field when required. 



A binocular instrument costs more than a single one. 

 If the prism which diverts part of the light-beams to form 

 a second image is good, an object viewed, say, with half-inch 

 power, should be defined sharply and clearly whichever 

 tube is looked through. If intended for use with high 

 powers, Stephenson's pattern is the best Before deciding 

 on having a binocular instrument, the person who is to ijse 

 it should ascertain whether his eyes are an optical pair. A 

 slight difference does not matter ; but, when one focus is 

 considerably further off than the other, a binocular is of 

 little use. "When both eyes match, the effect is excellent, 

 and fatigue from continuous observation is less felt. 



Beginnei-s 'are puzzled to know what is meant by the 

 angle of aperture of object-glasses, and what is the effect 

 of it. 



Let A be the front lens of an objective, B C two rays of 

 light making an angle, B D C. That is the angle of 



aperture, and it means that the objective is open to, and 

 can bring to a focus, rays of that degree of obliquity. 

 But it may be constructed to bring such rays at F G to a 

 focus, and their angle, as seen at E, is larger than that at 

 D. There are proportions between focal length and angles 



of aperture which give the best definition. This is often 

 exceeded for the purpose of showing dotted and lined 

 objects, such as diatoms ; but naturalists and physiologists 

 condemn the plan on several grounds. Such objectives 

 are less accurate, and have little paietration — that is, they 

 cannot show both the surface of object and layers a 

 little below it. They also can only work very close to 

 an object in proportion to their magnifying power, and 

 this is extremely inconvenient, especially when living 

 creatures are under investigation. When it is considered 

 that a good microscope is a permanent treasure in a house, 

 it is worth while to begin with one as good as can be 

 afforded. The most costly instruments are luxuries. All 

 they can do can be done with simpler and much cheaper 

 means, but it is not wise, if it can be helped, to stop shoit 

 of what are reckoned good second-class stands. The objec- 

 tives should be of the best quality, and only advanced 

 studeiits will do any good with powers higher than \ or ;.. 

 In all cases they and the microscope tube objectives should 

 be made with the universal screw. 



OUR TWO BRAINS. 



By Richard A. Proctoe. 



(Continued from p. 436.) 



THE remarkable phenomena presented in cases of dual 

 or intermittent consciousness appear to throw light 

 on the subject we are dealing with, because they can 

 apparently be interpreted in no other way (when all are 

 considered) than by the theory that the brain is double. 

 Let us consider a few cases of dual consciousness : — 



Brown-Sequard mentions the case of a boy at Notting- 

 hill who had two mental lives. Neither life presented an)"- 

 thing specially remarkable in itself. The boy waa a well- 

 mannered lad in his abnormal as well as in his normal con- 

 dition — or one might almost say (as will appear more 

 clearly after other cases have been considered) that the tiro 

 boys were qtiet and weU behaved. But the two mental 

 lives were ei t rely distinct. In his normal condition the boy 

 remembered nothing which had happened in his abnormal 

 condition ; and vice versa, in his abnormal condition he 

 remembered nothing which had happened in his normal 

 condition. He changed from either condition to the other 

 in the same manner. " The head was seen to fall suddenly, 

 and his eyes closed, but he remained erect if standing at the 

 time, or if sitting he remained in that position (if talking, 

 he stopped for a while, and if moving, he stopped moving) ; 

 and after a minute or two his head rose, he started up, 

 opened his eyes, and was wide awake again." While the 

 head was drooped he appeared as if either sleeping or falling 

 asleep. He remained in the abnormal state for a period 

 which varied between one hour and three hours ; it appears 

 that every day, or nearly every day, he fell once into his 

 abnormal condition. 



This case need not detain us long, but there are some 

 points in it which deserve more attention than they seem 

 to have received. It is clear that if the normal and 

 abnormal mental lives of this boy had been entirely distinct, 

 then in the abnormal condition he would have been ignorant 

 and — in those points in which manners depend on training 

 — ill-mannered. He would have known only, in this con- 

 dition, what he had learned in this condition ; and as only 

 about a tenth part of his life was passed in the abnormal 

 condition, and presumably that portion of his life not 

 usually selected as a s\iitable time for teaching him, the 

 abnormal boy would of necessity have been much more 



