108 THE MICROSCOPE. July 



197. — How may I hold sections of rock for grinding between glass 

 pkUes, with emery, and how shall I separate and clean them? — Nev- 

 ins. 



Cement the rock slices to the glass with hard balsam, using a 

 gentle heat. After grinding and polishing, remove the balsam 

 b}' soaking the glass and section in spirits of turpentine. Rock 

 sections are very fragile and in some cases it is better to grind 

 them on the slip which is to be used for the permanent mount^ 

 and not remove them, but wash away the debris with turpen- 

 tine and a brush. When clean add fresh balsam and cover. 



198. — Please give a satisfactoty process for separating and clean- 

 ing Foramenifera, etc., from chalk f — R. D. Nevins. 



Use a soft brush and water to rub down the chalk. Mix the 

 material with water and allow the shells to fall to the bottom ; 

 or, if these heavier portions are broken or not valuable, secure 

 the good portions while still suspended in the water, by pour- 

 ing them into another vessel and letting them subside. Ex- 

 amine the deposit as well as the floating material at times, with 

 the microscope, to determine their relative values. If the de- 

 sired shells are rough or covered with granular matter, shake 

 them in a test tube with four times their bulk of well cleaned, 

 heavy, sharp, white sand (prepared by washing and settling as 

 above.) After the shells are clean allow the sand to sink, which 

 it will do before the lighter shells. Pour off the latter into an- 

 other tube. Here the shells will sink before the lighter fluffy 

 material, and by a careful timing and several repetitions of 

 washings and settlings, the fluff may be poured away, and 

 clean shells secured. But few perfect shells from a considerable 

 mass of material should be expected. 



199. — What is meant by the standard tube length f I read of a ten 

 inch, nine and a quarter, eight and a half and six inch lengths, and 

 so on, and am confused ivhen I loish to use objectives by different mak- 

 ers. A reader. 



The standard tube length for American objectives is ten 

 inches, measured from the optical center of objective to the 

 optical center of eye piece. Optical centers are difficult to 

 locate, hence the clear directions by Mr. Spencer, to employ a 

 tube length of 9i inches, measured from the shoulder against 

 which the objective rests, to the eye surface of an inch ocular, 



