[26] _Pleuroscopes_.--As mentioned above the anterior commissure laryngoscope and the esophageal specula make very efficient pleuroscopes; but three different forms of pleuroscopes have been devised by the author for pleuroscopy. The retrograde esophagoscope serves very well for work through small fistulae. _Measuring Rule_ (Fig. 7).--It is customary to locate esophageal lesions by denoting their distance from the incisor teeth. This is readily done by measuring the distance from the proximal end of the esophagoscope to the upper incisor teeth, or in their absence, to the upper alveolar process, and subtracting this measurement from the known length of the tube. Thus, if an esophagoscope 45 cm. long be introduced and we find that the distance from the incisor teeth to the ocular end of the esophagoscope as measured by the rule is 20 cm., we subtract this 20 cm. from the total length of the esophagoscope (45 cm.) and then know that the distal end of the tube is 25 cm. from the incisor teeth. Graduation marks on the tube have been used, but are objectionable. [FIG. 7.--Measuring rule for gauging in centimeters the depth of any location by subtraction of the length of the uninserted portion of the esophagoscope or bronchoscope. This is preferable to graduations marked on the tubes, though the tubes can be marked with a scale if desired.] _Batteries_.--The simplest, best, and safest source of current is a double dry battery arranged in three groups of two cells each, connected in series (Fig. 8). Each set should have two binding posts and a rheostat. The binding posts should have double holes for two additional cords, to be kept in reserve for use in case a cord becomes defective.* The commercial current reduced through a rheostat should never be used, because there is always the possibility of "grounding" the circuit through the patient; a highly dangerous accident when we consider that the tube makes a long moist contact in tissues close to the course of both the vagi and the heart. The endoscopist should never depend upon a pocket battery as a source of illumination, for it is almost certain to fail during the endoscopy. The wires connecting the battery and endoscopic instrument are covered with rubber, so that they may be cleansed and superficially sterilized with alcohol. They may be totally immersed in alcohol for any length of time without injury. * When this is done care is necessary to avoid attempting to use simultaneously the two cords from one pair of posts. [FIG 8.--The author's endoscopic battery, heavily built for reliability. It contains 6 dry cells, series-connected in 3 groups of 2 cells each. Each group has its own rheostat and pair of binding posts.]