HE literature referring to this compound is usually more or less incomplete in regard to the details of its preparation ; but, unless it is prepared in the pure state, its use as a delicate reagent is somewhat impaired, Place sixty grams pulverized potassium ferrocyanide in a medium-sized evaporating dish, and pour on it a solution consisting of 150 grams concentrated nitric acid (sp. gr. 1.42) diluted with 100 cc. of water. Perform the operation under a hood or in a draft of air. When the evolution of gases has ceased, evaporate on a water-bath with occasional stirring until a few drops of the liquid, mixed with water in a test-tube, no longer give a blue

A prospective comparison between the Ohkuma' and Ishihara2 pseudoisochromatic (PIC) plates was carried out in a group of 400 patients attending a general ophthalmology practice. The sensitivity of the Ohkuma test was compared to the lshihara test, and the specificity of both was determined by reference to anomaloscopy as a gold standard.Both tests correctly identified the same group of 24 patients as having a red/green confusion axis, and the Ohkuma test was equally as sensitive as the Ishihara. The grading plates in both tests are unreliable, but the Ohkuma test is quicker, easier to administer, gives less ambiguous responses and has a clearer cut-off score for abnormality. On the basis of this experience the Ohkuma test is recommended as more appropriate for routine colour vision screening than either the 24 or 38 plate lshihara tests. Key words:Colour vision defects, lshihara plates, Ohkuma plates, pseudoisochromatic plates.The Ishihara plates constitute one of the most widely used screening tests for colour vision defects in ophthalmic practice. Although the pigments were chosen empirically, the test has been so successful that it is recommended as a screening tool by the International Civil Aviation Organisation. The sensitivity and specificity depends on the population under test and the cut-off level chosen. Hill and Aspinal13 report the probability of correctly classifying normal colour vision is 99% in both males and females, but the probability of correctly diagnosing an abnormal observer from a failed Ishihara test is 82% in males and 19% in females if the recommended cut-off is used. A lower cutoff improves the sensitivity, while a higher cut-off improves the specificity. These data suggest that ifthe Ishihara test is used with a cut-off at one error, the sensitivity will be over 99% but all positive results require a second highly specific test, such as anomaloscopy, to remove the inevitable false positives.The underlying principle of anomaloscopy is that if an observer is asked to mix a standard red with a standard green in the proportions necessary to match a test light of standard yellow, then the proportions of red and green accepted by the observer as a match for the yellow are informative about that observer's relative sensitivity for the red and green stimuli. The relationship between this test result and the absorption spectra of the anomalous photoreceptor pigments is complex, and at this stage the relationship to the underlying molecular genetics is ~n c e r t a i n .~ Nevertheless, the anomaloscope remains the gold standard for colour vision testing, with 100% specificity by definition.The variation in the redlgreen ratio accepted as a match is known as the matching range, which covers the full dynamic range of the instrument in a true dichromat. Normal trichromats accept very small variations in the redlgreen ratio around a mean value known as the mid-match point. Anomalous trichromats have a widened matching range and a shift in the mid-match point which is diagnostic of the col...

No abstract

BEFOREproceeding with the analysis, it may be desirable to ascertain the approximate specific gravity of the sample as follows:Weigh the can with its contents. Then remove contents, replace with water to the top, and weigh again. Weigh empty can (sufficiently dried) and ascertain weights of milk and water separately. Divide the weight of condensed milk by the weight of water as a standard, and the quotient is the specific gravity of the condensed milk (near enough for practical purposes).The specific gravity, which is not a matter of great importance, may vary from 1.27 to 1.37, according to the quantity of canesugar added, or according to the relative amounts of fat and albuminoid material in the milk used for condensation. That is, milk deficient in fat may show an increment in gravity irrespective of the amount of sugar added.As to the method of analysis: the contents of the can, together with any portions of crystallized sugar, which may adhere to the bottom of the can like a hard mealy mass, are transferred to a beaker and stirred vigorously until the mass iRead before the New York Section of the American Chemical Society, February 10,1899.

ACETANILIDE, PHENACETINE, QUININE SULPHATE, ETC. 933 14.6225 grams of a mixture containing 7.6037 grams of acetic acid and 7.0188 grams of benzene were placed in the tube described above and a few centigrams of water were added successively, the mixture being exposed to the temperature of a mixture of ice and salt, i.