This section is from the book "Every-Day Dishes And Every-Day Work", by E. E. Kellogg. Also available from Amazon: Larousse Gastronomique.
In preparing soups from grains, legumes, and vegetables, the material should be first cooked in the ordinary manner, in as small an amount of water as practicable, so as the more thoroughly to disintegrate or break it up. If the material be legumes or grains, the cooking should be slow and prolonged. The next step is to make the material homogeneous throughout, and to remove any skins or cellulose material it may contain. To do this, it should be put through a colander. The colander process being completed, a sufficient amount of liquid may be added to make the whole of the consistency of rather thick cream.
If the material is now cold, it must be reheated, and the salt, if any is to be used, added. The quantity of salt will depend somewhat upon the taste of the consumer; but in general, one-half teaspoonful to the pint of soup will be an ample supply. If any particular flavor, as of onion or celery, is desired, it may be imparted to the soup by adding to it a slice of onion or a few stalks of celery, allowing them to remain during the reheating. By the time the soup is well heated, it will be delicately flavored, and the pieces of onion or celery may be removed with a fork or a skimmer. It is better, in general, to cook the soup all that is needed before flavoring, since the delicate flavors are apt to be lost by evaporation in the boiling. When reheated,  add to the soup a quantity of cream as seasoning, in the proportion of one cup of thin cream for every quart or three pints of soup. Nut butter or nut meal may be used for seasoning, in the proportion of two table-spoonfuls to the quart of soup. Many soups are excellent without the addition of cream or other fat.
To avoid the possibility of any lumps or fragments in the soup, pour it again through a colander or a Chinese soup strainer into the soup tureen, and serve. It is well to take the precaution first to heat the strainer and tureen, that the soup be not cooled during the process.
The consistency of the soup when done should be about that of single cream, and equal throughout, containing no lumps or fragments of material. If it is too thick, it may be easily diluted with hot milk or water; if too thin, it will require the addition of more material, or may be thickened with a little flour or cornstarch rubbed to a cream with a small quantity of milk, used in the proportion of one tablespoonful for a quart of soup, - heaping, if flour; scant, if corn-starch. It should be remembered always to boil the soup five or ten minutes after the flour is added, that there may be no raw taste.