186 THE AMERICAN MONTHLY [Oct. 



eg'g's may not hatch for six or seven days ; the six-leg-g-ed 

 larvae may moult when three to four days old, and become 

 mature ; after pairing- a second moult takes place, lasting- 

 four to five days ; a third moult follows immediately, then 

 the eg-g-s are laid and the adults die ; in some cases there 

 is a fourth moult, but apparently without any further pro- 

 duction of egg"s. Accepting- Gerlach's estimate of fifteen 

 days as an average for each g-eneration of 10 females and 

 5 males, in three months' time the sixth g-eneration would 

 appear and consist of about 1,000,000 females and 500,000 

 males. — Bu. An. Industry. 



Intensity of Light. — At the Quekett Club Mr. Goodwin 

 said that in trying- some experiments with his little lamp, 

 he trie^ to fill the field with light without using-a bull's-eye 

 condenser, and he had adopted a method of adding- asmall 

 lens to the ordinary combination of the substage conden- 

 ser. By this means he had been enabled to bring the 

 lamp within three inches of the back combination of the 

 substage condenser, and in this way he got a full field of 

 light without in any way impairing the definition. He 

 thoug-ht this plan worth the attention of those who desire 

 to make the best use of their appliances. 



Mr. Nelson said he had seen the contrivance and tried 

 it and got one made for himself and though he had not ex- 

 haustively tested it he was assured of its merits. In the or- 

 dinary way the lamp must be put about 8 inches from the 

 back of the lens of the condenser for best effects. His 

 plan places a plano-convex lens of 5 inch focus in the 

 screen holder of the condenser, which enables us to bring- 

 the lamp up closer — to 3 inches instead of 8 inches which 

 is a matter of great importance when high powers are used. 

 The idea is simple and capable of development. 



Mounting Diatoms. — The best workers now use either 

 Styrax, liquid am bar or a mixture of balsam and mono- 

 bromid of naphthalin. Moller, Thum and Tempere all 

 use the latter a great deal, while Van Heurch prefers 

 liquid ambar. — D. B. Ward. 



