PROCURING OBJECTS. 55 



optical instrument makers. Two very fine needles mounted in small 

 handles are exceedingly useful in separating the various organs of in- 

 sects; or the forceps, the knife, and point K, Fig. 13, will be found 

 admirably adapted to aid the microscopist in his dissections, either 

 before or while the object is beneath the miscroscope. 



57. ANATOMICAL INJECTIONS. In the minute structure of the 

 organs and tissues of higher animals, the microscope has been of the 

 most essential service. The injection of these preparations is rather 

 the province of works on anatomy ; and I shall only here refer to a 

 new method of injection, which, from its minuteness, is more particu- 

 larly adapted to the microscope. 



M. Doyere has devised a method for obtaining minute injections 

 of the greatest utility in the examination of such structures beneath 

 the microscope. The process consists in causing to enter in the same 

 vessels, within a certain interval of time, two finely-filtered saline 

 solutions, which, by double decomposition, give an abundant and 

 opaque precipitate. The second solution is injected as soon as the 

 first has passed from the arterial system into the venous and lym- 

 phatic systems. 



M. Doyere has made a great number of experiments on the 

 subject, from which he is led to prefer, to all others, the chromate 

 of lead. He first injects a solution of chromate of potash; and 

 it is to be remarked, that the order of injection is a point not 

 to be neglected. A limpid solution of acetate or nitrate of lead 

 is then injected, and a beautiful yellow injection is the result. 

 A blue colour may be obtained by the precipitation of Prussian blue : 

 brilliant red, of iodide of mercury ; white, of carbonate or sulphate of 

 lead from the usual solutions. 



58. TEETH AND SHELLS. Sections of the teeth of various animals 

 are beautiful microscopic objects. The principal substance of the teeth 

 in almost all animals, is one called Dentin, characterised by minute 

 tubular passages permeating it in a direction from the centre to the 

 circumference. Considerable variation in the arrangement of these 

 tubula is found in different groups of animals, which enable naturalists 

 to determine, with great precision, by the microscopic examination of 



