Our visual perception depends on the reception of energy reflecting or radiating from that which we wish to perceive. If our eyes could receive and measure infinitely delicate sense-data, we could perceive the world with infinite precision. The natural limits of our eyes have, of course, been extended by mechanical instruments; telescopes and microscopes, for example, expand our capabilities greatly. There is, however, an ultimate limit beyond which no instrument can take us; this limit is imposed by our inability to receive sense-data smaller than those conveyed by an individual quantum of energy. Since these quanta are believed to be indivisible packages of energy and so cannot be further refined, we reach a point beyond which further resolution of the world is not possible. It is like a drawing a child might make by sticking indivisible discs of color onto a canvas.
We might think that we could avoid this limitation by using quanta with extremely long wavelengths; such quanta would be sufficiently sensitive to convey extremely delicate sense-data. And these quanta would be useful, as long as we only wanted to measure energy, but a completely accurate perception of the world will depend also on the exact measurement of the lengths and positions of what we wish to perceive. For this, quanta of extremely long wavelengths are useless. To measure a length accurately to within a millionth of an inch, we must have a measure graduated in millionths of an inch; a yardstick graduated in inches in useless. Quanta with a wavelength of one inch would be, in a sense, measures that are graduated in inches. Quanta of extremely long wavelength are useless in measuring anything except extremely large dimensions.
Despite these difficulties, quanta have important theoretical implications for physics. It used to be supposed that, in the observation of nature, the universe could be divided into two distinct parts, a perceiving subject and a perceived object. In physics, subject and object were supposed to be entirely distinct, so that a description of any part of the universe would be independent of the observer. The quantum theory (quantum theory: 量子论), however, suggests otherwise, for every observation involves the passage of a complete quantum from the object to the subject, and it now appears that this passage constitutes an important coupling between observer and observed. We can no longer make a sharp division between the two in an effort to observe nature objectively. Such an attempt at objectivity would distort the crucial interrelatioship of observer and observed as parts of a single whole. But, even for scientists, it is only in the world of atoms that this new development makes any appreciable difference in the explanation of observations.
17. The primary purpose of the passage is to
(A) discuss a problem that hinders precise perception of the world
(B) point out the inadequacies of accepted units of measurement
(C) criticize attempts to distinguish between perceiving subjects and perceived objects
(D) compare and contrast rival scientific hypotheses about how the world should be measured and observed（A）
(E) suggest the limited function of sensory observation
18. According to the passage, quanta with an extremely long wavelength cannot be used to give complete information about the physical world because they
(A) exist independently of sense-data
(B) are graduated only in inches
(C) have an insignificant amount of energy
(D) cannot, with present-day instruments, be isolated from quanta of shorter wavelength（E）
(E) provide an insufficiently precise means of measuring length and position
19. Which of the following describes a situation most analogous to the situation discussed in lines 9-13?
(A) A mathematician can only solve problems the solution of which can be deduced from known axiom.
(B) An animal can respond to no command that is more complicated syntactically than any it has previously received.
(C) A viewer who has not learned, at least intuitively, the conventions of painting, cannot understand perspective in a drawing.
(D) A sensitized film will record no detail on a scale that is smaller than the grain of the film.（D）
(E) A shadow cast on a screen by an opaque object will have sharp edge only if the light source is small or very distant.
20. The author uses the analogy of the child’s drawing (lines 17-19) primarily in order to
(A) illustrate the ultimate limitation in the precision of sense-data conveyed by quanta
(B) show the sense of helplessness scientists feel in the face of significant observational problems
(C) anticipate the objections of the those scientists who believe that no instrumental aid to observation is entirely reliable
(D) exemplify the similarities between packages of energy and varieties of color（A）
(E) disparage those scientists who believe that measurement by means of quanta offers an accurate picture of the world
21. The author implies that making a sharp division between subject and object in physics is
(A) possible in a measurement o object’s length and position, but not in a measurement of its energy
(B) still theoretically possible in the small-scale world of atoms and electrons
(C) possible in the case of observations involving the passage of a complete quantum
(D) no longer an entirely accurate way to describe observation of the universe（D）
(E) a goal at which scientists still aim
22. The author’s use of the phrase “in a sense” (line 34) implies which of the following?
(A) Quanta of extremely long wavelength are essentially graduated in inches.
(B) quanta of one-inch wavelength are not precisely analogous to yardsticks graduated in inches.
(C) Quanta of extremely long wavelength, in at least one respect, resemble quanta of shorter wavelength.
(D) quanta of on-inch wavelength and quanta of extremely long wavelength do not differ only in their wavelengths.（B）
(E) quanta of one-inch wavelength must be measured by different standards than quanta of extremely long wavelength.
23. According to the passage, the quantum theory can be distinguished from previous theories of physics by its
(A) insistence on scrupulously precise mathematical formulations
(B) understanding of the inherent interrelationship of perceiver and perceived
(C) recognition of the need for sophisticated instruments of measurement
(D) emphasis on small-scale rather than on large-scale phenomena（B）
(E) regard for philosophical issues as well as for strictly scientific ones