GENERAL METHOD. xxvii 



light of the clouds, which may be easily proved by altering the inclination of the mirror so 

 as to reflect the light of the clouds and the lamp alternately. 



It has been denied that this can be done, but it is as easily done as described. It may 

 be remarked, that the nearer the colom'ed glass is placed to the flame the less apparent effect 

 will be produced, i. e. the more will the yellow colour be perceptible, and vice versa. If the 

 field still appear yellow, the glass is not of sufficiently deep colom- ; if it appear blue, the 

 colour of the glass is too deep. The first method, or that of mixing some substance with 

 the combustible (oil, tallow, &c.) capable of evolving a light of the requisite tint to form 

 white with the yellow of the artificial light, would be far preferable to the latter method, 

 but we are not aware that the subject has been taken up by any one, or that anj^ experiments 

 have been made to cany out the idea. It would have two great advantages, viz. that there 

 would be no diminution of light, and that the entire apartment would be illuminated bv a 

 light equivalent to that of ordinary day. The second method has one objection, which 

 is, that it intercepts a large quantity of the light, so that in the examination of those objects 

 with high powers which require intense illumination, or where much of the light is arrested 

 by stops, it is decidedly objectionable. The advantages which the use of the blue glass 

 possesses are, that it softens the light very much, and that it enables the observer to discri- 

 minate between colours as in ordinarj'^ daylight. 



A few years after the publication of the above method, a patent was taken out for the 

 construction of lamp-glasses of a blue colour ; but they are of little service, merely slightly 

 softening the light, or intercepting a small proportion of the yellow rays. 



The proper way would be to " flash " the properly tinted blue glass upon one side of a pale 

 blue lamp-glass, so that by simply turning the glass round, the light might be transmitted 

 through either of the diff'erently coloured portions. 



II.— GENERAL METHOD OF DETERMINING THE STRUCTURE OF MICRO- 

 SCOPIC OBJECTS FROM THE APPEARANCES WHICH THEY PRESENT 

 UNDER VARIOUS CONDITIONS. 



Microscopic and histological appearance, structure and analysis. — Before proceedingto this, 

 let us define what is to be meant by the structm-e of a microscopic object. If we take a 

 piece of the free end of the finger nail, and examine thin transverse sections of it under the 

 microscope, we find them to present numerous shorter or longer dark and somewhat irre- 

 gular lines running nearly parallel to the surfaces. These appearances do not vary essentially 

 whether it be examined in the dr)' state, or immersed in water or oil of turpentine. 



But when it is moistened with solution of potash, and allowed to remain so for some time, 

 or the slide is gently heated, it becomes entirely resolved into a number of nucleated 

 cells ; and by watching the gradual action of the potash, it is easily seen that the cells were 

 originally flattened and arranged in layers, which layers produced the lined appearance 

 mentioned above (see the article Nails). Now which is to be considered as repre- 

 senting the structure of the nail, the first or the second of the above results ? Undoubtedly 

 the second. The expressions microscopic structure and histological structure are used very 

 indefinitely, and often synonymously ; but the former may very convenientl}'^ be restricted 

 to signif}^ the apparent structure as determined with the aid of ordinary mechanical means ; 

 whilst the latter may designate the true structure in relation to development. It may at 

 first sight appear very unnecessary to make any distinction between the two ; but it is 

 really very important, because most of the descriptions of the structure of bodies, at least 

 as given by English writers, refer only to their microscopic structure, nay, often to their 

 microscopic appearance only. These occupy but little time, and readily affbrd the basis of 



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