118 THE AMERICAN MONTHLY [May, 



BOYS' DEPARTMENT. 



Jack's Yisit to the Natural History Society.* 



By Dr. J. E. TAYLOR. 



One wet evening Willie Ranson got Jack to go to the Society just be- 

 cause there was nothing else to do. There was a short paper being 

 read on " Fish Scales," and a number of them were mounted for 

 microscopical examination, of course with a low power, say inch 

 and half-inch. Anything relating to fish or fishing was certain to gain 

 Jack's attention, therefore a better subject could not have been selected 

 to engage his notice. Besides, Jack had never yet even looked through 

 a microscope ! He felt a bit ashamed of this now ; but there were a 

 couple of microscopes present, and Jack determined to have a good 

 look through them. The scales of different sorts of British fishes were 

 on view. Of course, fish-scales are common enough ; but who would 

 think that each kind has its own pattern of scale, and that you could 

 tell a species offish by its scales? 



The paper showed that the scales of fishes were composed of the 

 same material, chitine, as the feathers of birds, or the hair and nails 

 of animals — a kind of substance onlv found in the animal kingdom, 

 and never in the vegetable ; that these scales are developed in little 

 pockets in the fish's skin, which you can plainly see for yourself when 

 a herring is scaled. They are arranged all over the fish's body like the 

 tiles covering a roof, partly overlapping each other, as is seen by one 

 part of the scale being often different from the other. 



Jack looked through the microscope and was delighted. He was 

 always a reverent-minded boy, and the sight broke on his mind like 

 a new revelation. How exquisitely chased and beautiful were the 

 markings, lines, dots, and other peculiarities ! Then the scales which 

 run along the middle line of the fish were shown him, and the ducts 

 perforating them, out of which the mucus flows to anoint the fish's 

 body, and thus reduce the friction of its rapid movement through the 

 water. The lad was half bewildered at the possibility of the new 

 knowledge. "■ Could anybody get to know about these things?" he 

 asked Willie, who told him of course he could, if he would only take 

 a little trouble. 



QUERIES. 



i. Does photomicrography constitute a crucial test for perfect achro- 

 matism ? If so, please explain why and how. 



2. Is a collar correction for an apochromatic with a compensating 

 ocular of any benefit? If so, how and why? 



3. Are apochromatics sensitive to tube length ? If not, why not? If so, 

 explain how. 



4. What is the process of obtaining dead black parts of the microscope 

 through the use of emery? 



* From '• Beginnings in Science at Mugby School," in the Popular Science Monthly for May. 



