142 THE MICROSCOPE IN THE 



the point which he desires specially to impress on his class has 

 been clearly seen and noted by each member of the class. 



But, how would all these difhculties vanish, if it were practic- 

 able to throw on the screen enlarged images of the objects of such 

 dimensions as would admit of all seeing them easily and suffi- 

 ciently distinct to make evident the special point that the 

 lecturer or teacher desired to explain. There would then be no 

 question of the powers of observation of the student, and there 

 would be no difference in the quality of the preparations. All 

 would see the same object, and the teacher would be able to call 

 special attention to the point under discussion, and he would be 

 in no doubt whether or not attention had been directed to the 

 actual or to some supposed detail. It cannot, therefore, be a matter 

 of surprise that many attempts have been made to attain this 

 desirable object, and we now have to consider these attempts, and 

 how they have been successful or otherwise, and what are the 

 difficulties to be overcome. 



As all objects, whether micro- or macro-scopic, are visible 

 solely by virtue of the light that they emit, intercept, or reflect, 

 we may in the first place consider what sources of light are avail- 

 able for our purpose. First, in point of efficiency, stands solar 

 light. The rays that issue from a source so distant are practically 

 parallel, and as such easily converted into converging or diverging 

 beams. Then, solar light is more intense than any other (I speak 

 under correction on this point). I find Dr. Young, in his treatise 

 on "The Sun," sets down the brightness of the sun's disc as 

 190,000 times that of the brightness of a standard candle. This 

 intense brightness of the solar light is a most important considera- 

 tion, and were it available at all times would leave us little to 

 desire so far as light is concerned. But in this country, at all 

 events, sunlight cannot be calculated on, and is entirely wanting 

 at those times when the lecturer and teacher are most likely to 

 want its help ; therefore, for the purposes we are considering, sun- 

 light may be set aside as not available. There is also this objec- 

 tion to sunlight when used in any concentrated form. The heat- 

 rays which accompany it are so numerous that I doubt whether it 

 be possible so far to absorb them as to admit of bringing delicate 

 preparations within their reach. 



