Glass, light transmittion.

Ordinary polished plate or sheet glass, when clean, transmits about 90 per cent of the daylight which falls upon it. In passing through the glass the light the light is slightly refracted, but this factor can usually be ignored.

Contrary to the соmmon belief a bard impervious finish does not remain clean though it can be easily cleaned by washing down. In the normal town atmosphere the daylight transmission will be reduced from the value of 90 per cent. This reduction depends upon atmospheric conditions, frequently on cleaning and whether the glass is vertical, sloping or horizontal. Certain special types of glass are made with one face ribbed prism form to collect high angle light, and by means of the prisms, to send the light into a room into which it would not otherwise penetrate. While these prismatic glasses can be useful, their value must not be overestimated. The resulting light often enters very neatly horizontally, which makes it of less value for illuminating horizontal surfaces. It must be remembered that unless the prismatic surface of this type of glass is so placed that it receives a good direct light from the sky, it cannot be of any value. It is not a point quite frequently overlooked. It should be noted here that the quality of light is as important as the quantity, and that one of the most important qualities for good vision is an absence of glare. This point must be kept in mind when using any type of diffusing glass in windows. With ordinary plain glass the general direction of light is downward, but as soon as the plain surface is broken up the light rays are scattered. The transmission of ultra violet rays is frequently a subject for discussion because of their benefit to health. Ordinary window class prevents-most of the ultra violet rауs from passing. To pass а high percentage of ultra violet rays a special type of glass has been made.

Clean vision through glass depends upon the type of glass, its cleanliness and the lighting conditions on each side of the glass. Ordinary sheet glass gives a certain amount of distortion, though it is considered quite satisfactory for most ordinary glazing. For undistorted vision the two sides of a piece of glass must bе ground and polished to produce parallel and plane faces - as it is done with polished plate glass.

EXERCISES:

I. Find out the Infinitives. In the function of the adverbial modifier of purpose.

II. Point out the suffixes of the following words:

name the parts of speech they form, translate them: impervious, normal, atmospheric, vertical, useful, prismatic, serious, transmission, frequently, reduction.

III. Translate the following sentences using the two ways translation:

Ехample: Чтобы пропускать 90 % света, стекло должно быть поставлено вертикально.

a) То transmit 90 % of light glass must be placed vertically.

b) Glass must be placed vertically to transmit 90 % of light,

1. Призматическое стекло используется для того, чтобы освещать помещения, в которые не проходит обычное освещение.

2. Для использования ультрафиолетовых лучей в госпита­лях применяется специальное стекло.

3. Для получения изображения стекло должно быть отполировано с двух сторон.

4. Листовое стекло употребляется для стекления (to glass) зданий.

IV. Answer the following questions:

1. How much light does ordinary glass transmit?

2. What is the purpose of making glass ribbed in prism form?

3. Does ordinary glass transmit ultra-violet rays?

4. What does good vision depend on?

5. What glass prevents most of the ultra-violet rays from passing?

6. Does ordinary glass give any distortion?

7. How should glass "be treated for undistorted vision?

V. Make up the detailed plan of the text.

MODEL: Glass to bе used for windows is usually produced in sheets.

GLASS (PART 2)