EXPERIMENT STATION BULLETINS 237 



pose is to state one or two principles underlying it. A lens convex on 

 both sides, or convex on one side and plane on the other will magnify 

 objects. If the double convex lens be used to magnify an object, and 

 another lens to magnify the image formed by the first lens, the object is 

 magnified highly. In figures, if the first lens magnifies one hundred times 

 and the second lens ten times, the image of the object seen through the 

 first lens would be one hundred times as large. Now, if the second ampli- 

 fies this image ten times, the object seen through both lenses would be 

 increased one thousand times. In this way the ability to see very minute 

 objects becomes possible. The lenses are mounted in a brass barrel, and 

 this is attached to a steady stand. This is the simplest form of a com- 

 pound microscope. It is through the microscope that the morphological 

 properties of bacteria are made known, and this is the only means avail- 

 able. 



STAINING BACTERIA. 



Din their ordinary condition3 bacteria are translucent and many times 

 are very diflicult to see. To overcome this obstacle to the proper study of 

 micro-organisms, stains have been called into requisition. They now form 

 a very important part in the biological study of bacteria. The stains used 

 are known as aniline stains, indirect products of coal tar. They corre- 

 spond to the diamond dyes in commerce and many of them are the same. 

 When desired for use they are dissolved in alcohol to saturation and this 



solution is diluted with water. 

 Method. ^ cover slip is evenly spread with a loopful of material and 



dried carefully over a flame. By this drying the bacteria are 

 fixed on the cover slip and are thus prevented from washing off. The 

 stain is then applied directly upon the prepared side of the cover slip by 

 means of a pipette and allowed to remain about a minute, when the sur- 

 plus stain is removed by water. The cover slip is now ready for examina- 

 tion under the microscope. 

 Me^tliods. Besides this simple staining, methods more complex are 



employed for the recognition of certain bacteria, for the study 

 of peculiarities, and for the satisfactory demonstration of spores and fla- 

 gella. Some bacteria will respond to one method of staining while others 

 will not, a fact which frequently allows one species to be distinguished 

 from another. The tubercle bacillus is stained by a method to which only 

 one or two others will respond readily. Inasmuch as these others are not 

 likely to be met under the same circumstances as the tubercle bacillus, 

 this method of staining becomes truly diagnostic. At other times there 

 are peculiarities inherent in certain bacteria which can only be sharply 

 illustrated by special stains. In the diphtheria bacillus are peculiarities 

 of the protoplasm which require a distinct stain to determine them. Spores 

 are questionable till they respond to a certain staining reaction. It con- 

 sists in making a penetrating stain by the addition of carbolic acid. By this 

 means, the spores as well as the vegetative cell may be stained. After the 

 spores are once stained they part with the stain as reluctantly as they took 

 it, but the color is easily removed from the vegetative form; consequently 

 after the spore is stained the color is readily removed from the vegetative 

 form leaving the spore colored and the vegetative form uncolored. An 

 ordinary stain is now applied to the vegetative form which is a contrast to 

 the color of the spore and which will not affect the color of the spore at 

 all. This results in having the spore of one color, say red, and the vegeta- 

 tive form of another color, say blue. A strong contrast of colors there- 

 fore exists and the spores are seen distinctly. In the demonstration of 



