Brook : SrM:\rARY of Methods of ^Ficroscopical Research. 9 



alcohol. This will give after filteration, a deep red liquid Tvhich is 

 ready for use. It is only necessary to leave the object in the staining 

 solution until the colour has quite penetrated. Of course, if the 

 object has not been preserved in alcohol, it must be allowed to steep 

 in it for some time before being transferred to the staining solution. 

 For small objects, such as very thin sections, small worms, Protozoa, 

 the lower Arthropoday &c., an immersion of quarter of an hour, some- 

 times even less, is usually sufficient. For larger objects, particularly 

 those which will be wanted afterwards for section cutting, and which 

 must be stained very dark, the time will vary from several hours to 

 several days, according to the nature and size of the object. In these 

 cases it is always best to use a large quantity of the staining liquid. 

 Thin sections, particularly of delicate objects, should be stained in a 

 very weak solution, one which has been used before is often useful. 

 The surplus dye must all be removed from the tissues by frequent 

 washing in 70 % alcohol, and this must be continued so long as the 

 alcohol comes away coloured. This washing process, with large 

 objects, takes a long time, and a considerable quantity of alcohol. 

 It may, however, be effected more quickly by using the alcohol warm. 

 The tissues appear to be not in the least injured by it. In this way, 

 only that portion of the cochineal which is chemically bound up in the 

 tissues remains, so that all diffuse colouring is avoided. Objects are 

 not often stained too dark, the affinity of the tissues for the colouring- 

 matter, being usually not so great as to interfere with the transparency 

 of the object by too great an aggregation of dye in the nuclei 

 or cellular substance. The colour which objects are stained by this 

 process, varies very much, and seems partly to depend on the reaction 

 of the tissues themselves, and partly on the presence or absence of 

 certain salts. Naturally, on account of the great variety of substances 

 existing in the cochineal insect in its dried state, extracts of the same 

 with different strengths of alcohol, will give different results. The 

 extract made with absolute, or 9U% alcohol, is quite light in colour, 

 and quite useless for staining purposes. As more water is added to 

 the alcohol, the tincture becomes stronger in proportion, and I should 

 recommend that obtained by 50 % or 60 % alcohol for staining purposes, 

 if easy penetration of the liquid was not an important qualification of 

 a staining medium : and the greater percentage of water such a 

 medium contains, the less suitable will it be in this respect. Each 

 separate extract gives, on further addition of stronger or weaker 

 alcohol, or even of water, a cloudiness and precipitate containing 

 material which is only soluble in alcohol of that particular strength. 



