FREE INSTITUTE OF SCIENCE 



53 



MENDEL AND HIS LAW 



only promptly killed but arrested in their processes, so that the condition in 

 which they were caught remained for examination no matter how long this 

 examination might be deferred. Hence arose many "killing and fixing" fluids. 

 Minute animals in water could be suddenly doused with such fluids, with the 

 result that life ceased instantaneously and the tissues became fixed in condi- 

 tions so compHcated and so easily got by repetition of the process, that it 

 seemed probable that these were the conditions during life. 



The materials of which cells are made are so transparent that, on looking 

 through the cells, it is very difficult to discriminate one ingredient from an- 

 other. Accordingly, there has arisen a process of staining to make the different 

 components of the cell evident, which has added much to the accuracy of micro- 

 scopic work. The principle of this staining can be easily understood if we 

 imagine we have in hand a piece of white woven goods, whose threads are 

 known to be made of mixed cotton, silk, and wool, and the proportion of whose 

 materials we desire to ascertain. It will not be difficult to find a dye which 

 will color the silk with a fixed and permanent color, but which will readily 

 wash out of cotton and wool. Supposing this dye to be red in color. By 

 dipping the cloth into such dye, allowing it to remain until the color has pene- 

 trated the fibers, and then persistently washing it, the silk threads will remain 

 red while the wool and cotton will be white as before. Now let us find a blue 

 dye, which is a fixed color for cotton, but is fugitive on wool and silk. Im- 

 mersing the cloth in this dye stains it blue throughout, but the subsequent 

 washing removes the blueness from the wool and silk, leaving the cotton blue. 

 In this patriotic piece of cloth we now have red silk threads, white woolen and 

 blue cotton. There is now no difficulty in discriminating between these three 

 materials even with the naked eye. Exactly the same principle is used in 

 staining the tissues of animals and plants for examination under the microscope. 

 It is quite possible to select the dyes to such effect that the cell wall may be of 

 one color, the general protoplasm of another, the nucleus in its general structure 

 of still a third, while certain bodies in the interior of the nucleus will come out 

 clearly among them all. 



The compound microscope has improved but little in principle since it 

 first came into use. It consists essentially of a magnifying glass in one end of 

 a tube, with another similar but smaller glass at the opposite end. These are 

 so arranged with reference to each other that the first forms an enlarged image 

 which the second again magnifies. Refinement of lens-grinding and added 



