Volume 28, Issue 4
October 1948, pages 107-149
pp 107-130 October 1948
The incidence of white-fly is highly variable and the maximum variation occurs in leaves. The variations between the clumps and the canes if and when they exist are of a much lesser magnitude. Therefore, increase in the number of leaves alone in a sample is pre-eminently effective in the reduction of error of an estimate.
In a field where the infestation is either high or fairly high (puparia per square inch of the affected area more than 5.00), Nested sampling has to be adopted so as to ensure the equitable representation of clumps, canes and leaves in a sample. In such a field, 10–13 per cent, of the 3 ft. units should first be selected and then Nested sampling done in each unit in that particular form of the alternatives, which gives the larger number of leaves in the sample, namely 2 clumps × 2 canes × 5 leaves per unit or 20 affected leaves equitably representing the clumps and canes in a unit as far as possible or 18–20 per cent, of affected leaves (i.e., about one in every five) equitably representing the clumps and canes in a unit.
In a plot of low or mild infestation, Nested sampling is not essential. Only the leaves as a whole may be subjected to random selection, other requirements being the same as mentioned under number (ii) above.
For a required error percentage, a medium infested field (puparia, 2.00–5.51 per square inch) requires a smaller sample size than that needed in a plot of high infestation or of an infestation which is fairly high. A difference in the sample size in the number of leaves by even about 75% gave the same efficiency. This suggested inherent homogeneity in variation occurring in mild form of infestation.
If in each leaf, 10 random inch units (about 25% of the total inch units) be taken instead of enumerating the whole leaf, the estimate of the incidence will be fairly reliable with a very slight additional error; but for practical consideration and working convenience, the complete enumeration of a leaf may be preferred to counting puparia in 10 random inch-units.
pp 131-143 October 1948
The data collected and the limits of refractive index and constant of genuine milk worked out in the last two papers have been utilised in the attempt to detect different types of adulteration.
Samples of milk of different grades of refractive index and constant have been systematically watered and their rate of fall of the values determined. By viewing the two determinations in conjunction with each other it has been found possible to detect in average samples dilution with water up to a minimum of about 5–8 per cent.
Watered buffalo milk designed to pass off as cow milk reveals itself by the disturbed relationship between the refractive index and constant. While the refractive index of such a sample is normal for cow milk the refractive constant will be found to be usually too high for that value of the refractive index.
Added skim milk or removal of fat is detected by the lowered values of refractive constant, which in average samples is reduced below the minimum for pure milk at about a minimum of 20–25 per cent. addition. With samples of very high refractive constant, however, considerably larger dilutions (up to 50 per cent.) are possible.
Attempts at normalising the density of watered milk by adding sugar are detected up to about 10 per cent. added water in cow, and about 12–15 per cent. in case of buffalo milk samples.
Thus a simple determination of the refractive index and density of milk serves to detect a number of common types of adulteration of this product.
pp 144-149 October 1948