xiv INTRODUCTION 



are that the slide will be ruined, even though it does not show 

 injury. Microscope clips must not be placed over the cover 

 glass for they, too, will cause movement of the mounting medium 

 and irreparable injury to the preparation. 



Do not attempt to use high power lenses on thick preparations 

 such as those of trematodes, tapeworm proglottides and the like — 

 the proper manipulation of the mirror and the iris diaphragm 

 are important factors in bringing out detail under the low power 

 lenses. 



Use of Oil Immersion Lenses. — In the study of parasitic 

 protozoa it is often necessary to use the oil immersion objective. 

 This is, as its name implies, an objective whose front lens is 

 immersed in a special cedar oil of the same refractive index. As 

 it has a very short working distance it can be used only on prep- 

 arations having very thin cover glasses or on dried uncovered 

 films with only the oil. Special precautions must be observed 

 to prevent injury of either the lens or the preparation. A small 

 drop of oil is applied directly to the lens or placed on the prepara- 

 tion and the lens lowered until it is in contact with the fluid, 

 the observer watching closely meanwhile with the head at the level 

 of the stage. Focus cautiously — if there is apparent movement 

 of tissues in a balsam mount it is a danger signal that the cover 

 glass or the layer of balsam is too thick and the lens must immedi- 

 ately be focussed upwards. 



Dissecting Microscopes. — The modern binocular dissecting 

 microscopes are a great aid to the student of parasitology. An 

 equipment of lenses giving a range of magnification of from 

 seven to thirty times, is the most satisfactory. The higher 

 magnifications are of little use in general work. 



The student, however, must not get the idea that these are 

 essential for his work. Most of the anatomical details discussed 

 in the following pages were worked out long before the binocular 

 dissecting microscopes were invented. Some simple type of 

 dissecting stand, even if only the tripod with a lens which is 

 focussed by screwing up and down in the metal frame can be 

 provided in any laboratory. Skill in dissection of minute objects 

 under the ordinary compound microscope is readily acquired 

 and even today is an essential equipment of the student of zoology. 



Moreover, every student of the subject should own and habit- 

 ually use a good magnifier, mounted in a folding pocket case. 

 While the higher-priced lenses are of course preferable, very good 



