344 THE AMERICAN MONTHLY [Dec 



justmen^ is of the micrometer screw type, in which, how- 

 ever, a pointed rod impinges upon a hardened steel plate, 

 which is itself attached to the limb of the microscope and 

 works upon a triangular upright rod. In the larger model 

 the milled head of the line adjustment is graduated and 

 furnished with a folding pointer. The body tube is de- 

 signed for use with objectives corrected for the Continen- 

 tal length of tube, but carries a draw tube capable of va- 

 riation from 140 to 200 millimeters. The stage is square 

 and the upper surface is faced with ebonite. In the larger 

 model it measures 4x4 inches. In the least expensive 

 model a ring, of the Society size, is fitted beneath the 

 stage to carry the iris diaphragm and condenser. The lat- 

 ter is especially arranged to fit above the iris diaphragm, 

 and the arrangement is both effective and cheap. In the 

 larger models, or those fitted with more elaborate sub- 

 stage arrangements, the usual form of Abbe condenser is 

 provided for in this instrument. Messrs. Beck make their 

 "London" microscope in two sizes. The smaller size with 

 sub-stage ring and iris diaphragm, but without objec- 

 tives, eyepieces, or condenser, costs, with mahogany case, 

 only $16.00. The addition of a swing-out and spiral fo- 

 cussing snb-stage increases the price to $19.50. The larger 

 model, similar to the last described, costs ;?25.50, or with 

 rack and pinion foinissing sub-stage $31.60. The necessa- 

 ry eyepieces cost $1.25 each, the condenser in its sim- 

 plest form $2.40, whilst the objectives are of Messrs. 

 Beck's well-known and moderately priced series. 



Development of Balanus. — On the rocks of the south- 

 ern and western coasts of England, when the tide is out, 

 we observe that their surface is roughened up to a cer- 

 tain level with an innumerable multitude of brownish 

 cones. Each appears as a little castle built of strong 

 plates that lean towards each other but leave an orifice 

 at the top. Within thi« opening we see two or tliieo other 

 pieces joined together in a particular manner, but capa- 



