SLIDES AND COVERS. 793 



boiling sometimes suflBces, but it is always preferable to add some of 

 the strong acid, and boil the whole again for a few minutes, so as to 

 dissolve any vegetable or animal substances remaining. As the 

 siliceous covering is very thin, and easily broken by a sudden change 

 of temperature, care must be taken in washing away the acid, either 

 to use boiling water or to allow the Diatoms in the test-tube to cool. 

 When a sufficient supply of pwre distilled water can be easily got, it 

 alone ought to be used for washing them ; but, when that is not the 

 case, ordinary water may be employed for the first washing, but the 

 after washings must be all made with distilled water until the acid is 

 got rid of. After being thoroughly washed, the Diatoms are kept in a 

 small test-tube with some distilled water. In taking the specimens 

 from the test-tube, in order to put them on the slide, a pipette or 

 dropping-tube is employed, having a bore of about tVth to Tirth of an 

 inch at its lower end. 



Mr. Jackson remarks that it is desirable that no object submitted 

 to higher power than a quarter-inch objective of 75° aperture should 

 ever be mounted under a cover thicker than Tiirth of an inch ; if the 

 aperture exceeds 120°, the best thickness for the cover is ^rirth of an 

 inch.* Glass of this thickness can easily be cut with a good writing 

 diamond, when laid on a piece of plate glass.t To clean the covers 

 it is recommended to put them in strong sulphuric acid for a day or 

 two, and then wash them repeatedly with water ; after that to place 

 them, a few at a time, on a tightly-stretched clean cambric handker- 

 chief, and to rub them very gently with another handkerchief on 

 the finger. They should then be removed to a clean box, with 

 forceps, and carefully kept from dust and from contact with the 

 fingers. The covers should be sorted according to their thickness, 

 and this is done at once by Eoss's " lever of contact," which consists 

 of a long slender index, having a projecting touch near the centre 

 of motion, which is kept in'contact with a plane surface by means of 

 a spring. When a piece of glass is inserted under the touch, the 

 index points to the thickness on a graduated arc. The thickness 

 may also be measured in the usual way by placing a fragment in the 

 pliers, with the edge upwards, under the microscope, armed with an 

 inch object-glass and an eye-piece micrometer. J 



Works on the Microscope. — The following works may be con- 

 sulted by the student : — Carpenter, The Microscope and its Kevelations ; 



* On account of the brittleness of tlie glass, covers thinner than l-140th or l-160th of 

 an inch are, in the hands of most manipulators, practically useless, as they break by the mere 

 wiping or mounting, and glass l-160th of [an inch is not too thick either for Smith and 

 Beck's l-6th object-glass with 100° of aperture, or Ross's 1-Sth with 156° of aperture ; but 

 when dry mounting is adopted, the object ought to be arranged on the under side of the 

 cover, thus bringing it as near the lenses as possible. 



t Quekett on the Microscope. 2d Edit. p. 265. 



} Quarterly Journal of Microscopical Science, i. 141. 



