Question 27.—­Account for the delicate shades of colour sometimes seen on the inside of an oyster shell.  State and explain the appearance presented when a beam of light falls upon a sheet of glass on which very fine equi-distant parallel lines have been scratched very close to one another.

Answer.—­The delicate shades are due to putrefaction; the colours always show best when the oyster has been a bad one.  Hence they are considered a defect and are called chromatic aberration.

The scratches on the glass will arrange themselves in rings round the light, as any one may see at night in a tram car.

Question 29.—­Show how the hypothenuse face of a right-angled prism may be used as a reflector.  What connection is there between the refractive index of a medium and the angle at which an emergent ray is totally reflected?

Answer.—­Any face of any prism may be used as a reflector.  The connexion between the refractive index of a medium and the angle at which an emergent ray does not emerge but is p 183totally reflected is remarkable and not generally known.

Question 32.—­Why do the inhabitants of cold climates eat fat?  How would you find experimentally the relative quantities of heat given off when equal weights of sulphur, phosphorus, and carbon are thoroughly burned?

Answer.—­An inhabitant of cold climates (called Frigid Zoans) eats fat principally because he can’t get no lean, also because he wants to rise is temperature.  But if equal weights of sulphur phosphorus and carbon are burned in his neighbourhood he will give off eating quite so much.  The relative quantities of eat given off will depend upon how much sulphur etc. is burnt and how near it is burned to him.  If I knew these facts it would be an easy sum to find the answer.

1881.

Question 1.—­Sound is said to travel about four times as fast in water as in air.  How has this been proved?  State your reasons for thinking whether sound travels faster or slower in oil than in water. p 184

Answer(a).—­Mr. Colladon, a gentleman who happened to have a boat, wrote to a friend called Mr. Sturm to borrow another boat and row out on the other side of the lake, first providing himself with a large ear-trumpet.  Mr. Colladon took a large bell weighing some tons which he put under water and hit furiously.  Every time he hit the bell he lit a fusee, and Mr. Sturm looked at his watch.  In this way it was found out as in the question.

It was also done by Mr. Byott who sang at one end of the water pipes of Paris, and a friend at the other end (on whom he could rely) heard the song as if it were a chorus, part coming through the water and part through the air.

(b) This is done by one person going into a hall (? a well) and making a noise, and another person stays outside and listens where the sound comes from.  When Miss Beckwith saves life from drowning, her brother makes a noise under water, and she hearing the sound some time after can calculate where he is and dives for him; and what Miss Beckwith can do under water, of course a mathematician can do p 185on dry land.  Hence this is how it is done.