APOCHEOMATIC OBJECTIVES. 11 



perfection as to leave little to be gained until some method of 

 further increasing the aperture has been worked out. 



It has occurred to the writer that media having a higher 

 refracting index than cedar-oil, might be used with advantage 

 with immersion objectives constructed for the purpose. 



It is a fact well known to opticians that the extreme marginal 

 rays with glasses of wide aperture give no image ; experience 

 proves, however, that there is a decided gain in increasing the 

 aperture even up to N.A. 1*5. 



Is not this due to the fact that the last "i or more of aperture 

 is never utilised, and although we may get no more light through 

 the lens than would give an aperture of i"52, by using an immer- 

 sion medium of i'6 or more, might we not so compress the cone 

 of rays passing through as to make the marginal rays up to the 

 extreme limit of i"52 enter the front at a sufficiently favourable 

 point to be effective ? 



The Tarantula at Home. — Herr von Bergso, in a recent work 

 has given some interesting data respecting the habits of the Taran- 

 tula, Lycosa tiireiitiiia, whose nests he has traced and examined 

 on the Roman Campagna. He found that the nest, which was 

 well rounded and smooth, was approached by a tunnel which, 

 after running about a foot straight down below the surface of the 

 ground, made a sudden short turn before it finally descended for 

 about another foot into the spider's abode. The entrance to the 

 tunnel is concealed by an arched covering made by the interlacing 

 of grasses and leaves. The eggs are enclosed in a spun bag, and 

 the young appear in the autumn, when they immediately seat 

 themselves on the body of the mother where they remain till 

 about April, neither parent nor offspring seeking food during their 

 hybernation. As many as 291 individuals were on one occasion 

 removed in February from the body of an emaciated tarantula. 

 The superstitious error of assuming that the bite of the animal 

 induces an irresistible desire of dancing is due to the fact that 

 dancing having been originally employed as a remedy against the 

 poison, which is believed to be eliminated by profuse perspiration, 

 the action of the poison was confounded with the means of its 

 eradication. 



