ANILINE DYES — LEIRIND 435 



capillaries and was a pioneer in the study of the microscopic anatomy 

 of plants and animals; Kobert Hooke, who first described compart- 

 ments in cork which he called cells, thus introducing this word into 

 the language of biology. Hooke also published the first serious sci- 

 entific monograph on microscopy. Another of these early workers 

 was Jan Swammerdam, who performed incredible dissections of 

 insects under the microscope and devised the techniques of micro- 

 injection and micromanipulation. Perhaps the greatest of the classi- 

 cal microscopists was Antony van Leeuwenhoek, who first saw bac- 

 teria and protozoa, saw the blood pass through the capillaries from 

 arteries to veins, described spermatozoa, and was also the first to use 

 a coloring agent to stain tissue for observation under the microscope. 

 From the time of the death of Leeuwenhoek in 1723 to about 1830 

 advances in microscopy were sluggish. One reason was that micro- 

 scopes were so crude and their lenses so poor that few persons were 

 willing to take the trouble to use them. The principal defect in the 

 lenses was chromatic aberration. By 1830, however, crown and flint 

 glass was available, and this glass made possible the development of 

 lenses, especially in combinations, in which chromatic aberration was 

 eliminated. With the aid of achromatic lenses new advances were 

 made. The microscopic structure of plants and animals began to be 

 better undestrood, and in 1839 Schleiden and Schwann summarized 

 the observations of many workers and announced the cell theory. His- 

 tology, cytology, and embryology began to emerge as sciences. Nev- 

 ertheless, for technological reasons, progress was again limited. Most 

 of this early work was done with the use of low-powered lenses and 

 weak illumination and without the use of stains. Thus it was that in 

 the middle of the nineteenth century, Ferdinand Cohn, professor of 

 botany in Breslau, wrote : 



As long as the makers of microscopes do not place at our disposal much 

 higher powers, and, as far as possible, without immersion, we will find ourselves 

 ... in the situation of the traveller who wanders in an unknown country at the 

 hour of twilight at the moment when the light of day no longer suffices to 

 enable him clearly to distinguish objects, and when he is conscious that, not- 

 withstanding all his precautions he is liable to lose his way. 



Cohn's complaint was soon to be answered. The production of the 

 substage condenser and the development of homogeneous immersion 

 lenses (unavailable in Cohn's day) led to the tremendous improve- 

 ment in the illumination of objects observed under the microscope. 

 Simultaneously staining techniques were introduced, and they soon 

 became indispensable in biological and medical research and in medi- 

 cal diagnosis. 



The early history of biological staining is, as a matter of fact, still 

 quite confused, and it is foolhardy for anyone at present to give more 



