The utilization of urea in the bovine rumen. 1. Methods of analysis of the rumen ingesta and preliminary experiments in vivo.

visible light from the source. Vertical partitions (12) separate off a central compartment of the box and serve as supports for the three mirrors which it contains. Two of these (13) lie at right angles to each other in vertical planes, while the third (14) makes an angle of 45° with the horizontal. Fluorescent light from the two test tubes enters the central compartment through square apertures (15); it is reflected by the vertical mirrors on to the oblique mirror and thence upwards into the eyepiece (16) through a hole in the top of the box. The base of the eyepiece tube contains a filter (17) which cuts off practically all light except the. wave-lengths produced 'by the fluorescence of porphyrin. Interposed between the standard and the aperture (15), are the two glass screens (18); the dark end of one is above and of the other below, so that where they overlap they produce a uniformly shaded field. The screens are mounted in slides (19) which can move up or down a vertical guide (20). Each is held at the desired level by a pin (21) which enters a spiral groove in a vertical cylinder (22). The spiral is in one case right-handed and in the other left-handed. The two cylinders are fixed one above the other to a spindle (23) carried in bearings in the t,op and base of the box; a knurled head and a dial divided into a hundred divisions are mounted on the spindle above the top of the box. The spiral groove is of such a pitch that the whole range of movement of the slides is achieved by slightly less than one complete revolution of the spindle, and, as the grooves are rightand left-handed, equal and opposite movements are given to the two slides by a given degree of rotation. The slides are of such a length that their less dense ends are separated by a gap of about J in. when the dial knob is tt zero, so permitting unobstructed light to pass in this position from the right-hand tube to the mirror for comparison with the tube on the left. This arrangement has been found convenient for certain determinations in which a blank had to be evaluated. The inside metal work of the apparatus is painted black and the tube of the eyepiece is lined internally with matt surfaced black paper.